Both the Paladin and the Warpriest are combat-oriented divine casters, but what I want to know is, from an optimizer's perspective, what is the difference between the two classes in practice? What ways do the classes deal with challenges in-combat, and how do they deal with out-of-combat encounters? In asking this question, I am trying to determine the differences and merits associated with two classes that appear to be trying the fill the same niche in a way.
The warpriest gets higher-level spells, and is therefore better.
That’s really all it takes. That’s really all there is to say. Barring extremely weird cases where a spell list is very special (3.5’s healer and Pathfinder’s summoner come to mind), looking at the highest-level spell available to a class is a quick way to determine a class’s relative power. Magic is everything in this game.
The best thing about the paladin, from an optimizer’s perspective, is divine grace. That is very good. Might even be worth multiclassing for, in some cases (though probably not as a warpriest since Charisma would not be your first focus). But ultimately, “higher-level spells sooner” is a far more powerful class feature than anything the paladin has—including divine grace.
Beyond divine grace, lay on hands is OK, ish, but easily replaced by a wand of cure light wounds in most situations. In special situations, where the mercies become important, a wand of lesser restoration covers most of it; that’s pricier but will be used less often. These wands are ultimately not quite as good as lay on hands, but they’re close enough that lay on hands is a pretty low-value class feature.
Smite is decent in Pathfinder, but you get so few of them that I’m not super-excited about it. The fact that the warpriest gets several free quickenings a day from fervor easily outpaces smite—you get more uses of fervor, and you can buff yourself with things that lead to similar or better damage than smite does.
And the divine bond comes out to basically “pretty good, but nothing amazing.”
On the warpriest side, you have better spells, and you have the ability to cast them quicker. Fervor is a very, very good class feature. Quicken Spell is a +4-spell-level metamagic for a reason. And the cleric spell list is extremely well-positioned to take advantage of that ability.
Throw in some bonus feats and some free money from the sacred weapon and armor, and you’re in a pretty good spot.
An exception, of sorts
Paladins are in a pretty good place to start working on some Charisma optimization, which is far more “open” to optimization than Wisdom. Divine grace is a big part of that. You can add to that with starknives, which can get Charisma added to attack and damage through Divine Fighting Technique. And there are a number of ways to get Charisma to AC. Having one ability cover attack, damage, AC, and all of your saves is really very nice, and I’m pretty sure you could not do the same with Wisdom.
But there’s a problem with this: that Divine Fighting Technique is associated, in Golarion, with Desna, a CG goddess. Divine Fighting Technique requires that you match your deity’s alignment, and a paladin cannot do that. So you can only do this if playing in a campaign that associates it with an LG deity, or else relaxes alignment requirements in general (e.g. allows a non-LG paladin or Divine Fighting Technique without a matching alignment).
And even when that happens, paladin provides only a foundation; you wouldn’t be staying with the class most likely. Instead, you would most likely be an “oradin” rather than a paladin: taking two levels of paladin for divine grace, and then switching to oracle for one of the Cha-to-AC mysteries and the far-superior spellcasting is strictly superior.
From an optimizer's perspective, here's the similarities with comparisons. I am ignoring anything not part of the base class's abilities, or that is a choice (particular Blessings may make certain parts of the Warpriest better).
Paladin's Lay on Hands vs. Warpriest's Fervor
Lay on Hands starts at Level 2 and advances every 2 levels to a maximum of 10d6. Fervor starts at Level 2 and advances every 3 levels to a maximum of 7d6. So, in terms of being a self-healing tank, the Paladin is numerically superior due to the higher amount of damage they can heal. Beyond healing, Fervor allows the Warpriest to effectively use Quicken Spell for a use of Fervor while Lay on Hands added to Mercy can heal any number of nasty effects the party may run into.
As an optimizer looking to build Paladin, I would keep their secondary roles as both self-healing tank and combat medic in mind. As an optimizer looking to build Warpriest, I would look to capitalize on a relatively low-cost Quicken to quickly cast clerical buff spells on the party or myself, saving the healing for when a party member is about to die.
Divine Bond vs. Sacred Weapon
Divine Bond has two options: a mount and a buffed weapon. I'll be comparing the buffed weapon only for an apples-to-apples comparison.
Divine Bond starts at Level 5 and advances every 3 levels to a maximum of +6, with 10 options for special abilities. Sacred Weapon starts at level 4 and advances every 4 levels to a maximum of +5, with up to 11 options for special abilities (9 if neutral on any axis). This makes Divine Bond the numerically superior buff, though it does not start quite as early. In addition, Paladin gets their buff for 1 minute per Paladin level per day, while the Warpriest gets only 1 round per Warpriest level per day. Both are Swift actions.
As an optimizer, both effects serve the same function in combat with near-identical functionality out of combat.
BAB, Saves, Spells, and Skills
The Paladin is a Tier 1 BAB class with good Fortitude and Will saves, added to Tier 3 spellcasting off their own list. The spell list has some good options, but nothing incredibly noteworthy or worth expending a standard action for in combat as a martial character.
The Warpriest is a Tier 2 BAB class with good Fortitude and Will saves, added to Tier 2 spellcasting off the Cleric list. Their spell list includes some of the best buffs and debuffs in the game, several of which are worth expending a use of Fervor to cast as a Swift action on yourself.
From an optimizer's perspective, I would skew both characters towards martial ability for a basic build, with Paladin also getting 14 Charisma and Warpriest getting 16 Wisdom.
Paladin, outside of the Charisma requirement, needs to focus entirely on one of the low-feat combat options, so either two-handed weapons or archery. This is greatly assisted by Smite Evil, making them very strong front-liners or ranged strikers. Outside of combat, they have access to at-will Detect Evil, so having that basic level of insight is always helpful. Skill-wise, due to their higher than average Charisma, with a few points in Diplomacy they can make an decent face, solving problems with their words rather than their weapons. Not as good as a Sorcerer or Bard, and they are far from being a skill monkey, but solid nonetheless.
Other than the Wisdom requirement, Warpriest has more good options than just THW or archery with its bonus feats. Their first combat round should be a party-wide buff followed by a self-buff, after which they join the pure frontliners. Outside of combat, they have access to all the same spells as the Cleric, albeit slower and only the first six levels, making them strong utility casters. Beyond this, unfortunately they do not have the Cleric's ability to prepare spells more than one time per day, limiting their versatility somewhat. Skill-wise, due to their higher than average Wisdom, with a few points in Sense Motive, Perception, and Survival they can make an decent lie-detector and perceiver, solving problems through insight that others might have had trouble with. Not as good as a Druid or Cleric, and they are far from being a skill monkey, but solid nonetheless.
I didn't include Divine Grace here, which makes the Paladin even stronger in combat against mages. I also didn't include Channel Energy, which is generally an inefficient use of resources, and is largely the same between the two classes. Overall, I would say the Paladin is a dedicated combat class with strong buffing and battlefield medic utility, while the Warpriest is a dedicated buffing and battlefield medic class with strong combat utility. For a more precise idea of how each would handle a given situation, here's a few examples:
The Paladin and Warpriest are facing down a powerful martial enemy. The Paladin draws his sword, declares Smite Evil, and charges in to deliver a painful experience, all the while buffing the allies around him. The Warpriest casts a buff on himself, then casts a buff on the party, then on the next round draws his sword and charges in.
The Paladin and Warpriest notice that there is an enemy spellcaster. The Paladin charges, trusting his Divine Grace to protect him. The Warpriest casts a buff against the kinds of spells the enemy is throwing around, and then charges in to cut him down.
The Paladin and Warpriest are out of combat and attempting to purchase new weapons from a less-than-honest merchant, charging too high a price. The Paladin attempts to negotiate the price down automatically and ends up paying about what is fair while the Warpriest sees the shopkeeper's intentions and goes to another merchant.
The Paladin and Warpriest see that one of their allies was hit with a debilitating status condition. The Paladin hopes that he picked that condition as one of his Mercies, while the Warpriest hopes that he picked the right spell to cure that condition. If neither has the ability to cure it immediately, the Paladin needs to rely on another party member or NPC to help while the Warpriest presumably has the Heal skill and can keep them comfortable until the next day when they prepare their spells again and almost certainly have the right spell somewhere on their list to cure the victim.
I don't seee the Paladin and Warpriest as filling the same niche. As an optimizer, I would pick a Paladin for a party without a frontliner, but a Warpriest for a party with a frontliner. The Paladin is very much the hero leading the charge, while the Warpriest is more the right-hand-man of the hero leading the charge, who could lead the charge himself if called upon to do so.
The one glaring difference between the two is the alignment restriction on the Paladin. A Paladin must be Lawful Good, but a Warpriest can be any alignment. This can open up any number of combat-oriented divine options that just aren't available as a Paladin.
On a related note, the Paladin is a knight in shining armour, a paragon of both law and good. The Warpriest is a more combat-oriented priest, and so can fill a number of different character concepts that just wouldn't be suite by a cleric alone, such as a stealthy worshipper of a god of murder, or a combat-oriented worshipper of a trickster god.
From an optimization point of view, there are significant differences.
Paladin gets different spells to cleric, which are based around shutdowns and immunities and double damage sometimes where cleric gets the raw numbers
Paladin gets an animal companion and a warpriest gets enhancement bonuses to armour and weapon.
Paladin gets Smite a few times per day. Warpriest gets swift action small self-heals.
Overall warpriest is going to have higher atk/ac/damage numbers from cleric buffs, but a paladin will have higher saves and higher burst damage as well as an animal companion/mount that will probably put his damage higher than the warpriest.
For high op purposes, the usefulness probably looks like this -
Normal Cleric >>> Paladin > or = Warpriest.
As far as I can tell, there is never a reason from an optimization perspective to ever take warpriest when you could instead take cleric.