In Unearthed Arcana, there is a set of optional rules for Spontaneous Divine Casters. Under the assumption that these rules are used and that any Favored Soul fluff is ignored, is there any mechanical reason to play a Favored Soul instead of a Spontaneous Divine Caster Cleric? After all, the Favored Soul lacks some fantastic Cleric abilities like Turn Undead.
Actually, the Spontaneous Divine Caster is a somewhat watered-down Favored Soul.
The Favored Soul gains a number of minor features that the Cleric lacks:
- All Good saves (the Cleric has a poor Reflex save).
- Deity's Weapon Focus & Weapon Specialization.
- 3 Energy Resistance (10).
- Damage Reduction.
It also has a few major advantages:
- More spells known: a 20th-level Favored Soul knows 9 Cantrips, 6 1st to 7th level spells, 5 8th level spells and 4 9th level spells; compare to the Spontaneous Cleric and its 9 Cantrips, 5 1st/2nd level spells, 4 3rd to 5th level spells and 3 6th to 9th level spells.
- More spell slots: a 20th-level Favored Soul has 6 spell slots of each level, a 20th-level Cleric only 5 1st to 5th level spell slots and 4 6th to 9th level spell slots, plus 1 Domain spell slots solely for Domain spells (so not as flexible).
And of course, the spellcasting ability is different: Charisma vs Wisdom. It is easier to boost Charisma and it as a different set of skills associated, so it opens up different possibilities.
This does not necessarily mean that a Favored Soul is necessarily better than a Spontaneous Cleric:
- The wealth of published material for Domains and Turn Undead is just staggering.
- The Favored Soul uses Wisdom (still) for spellcasting DC, so is not as SAD as the Cleric.
- The Favored Soul is one level behind in spell levels, and only gains the minor features somewhat late.
It does offer, though, a compelling alternative. For straightforward play, without optimization research, the Favored Soul is strictly more powerful than the Spontaneous Cleric.
Nothing “rendered the favored soul redundant” since it never had much of a purpose to begin with
Having your ability score requirements split between Wisdom and Charisma is a large disadvantage. Spontaneous spellcasting is a huge disadvantage. Having your spells delayed a level is a massive disadvantage. And spells are the most important things in a spellcaster’s life—having worse spellcasting makes you a worse spellcaster, period. From the get-go, there was absolutely nothing a favored soul could do that a cleric couldn’t do better.
If you were dead-set on spontaneous spellcasting, sure, spontaneous cleric is vastly superior to the favored soul, so that’s another nail in the favored soul’s coffin. After all, the spontaneous cleric shares only one of the three large disadvantages I listed in the first paragraph, so it’s better. Having slightly more spells known or spells per day doesn’t begin to compare to simply having spells a spell level higher. But you’re still better off with the regular cleric than you are with either of the others.
The non-spellcasting class features of any of these classes are very distant seconds in terms of their significance to the overall abilities of the class. But even here, two or three domains and the ability to turn/rebuke undead is better than everything the favored soul gets: the bonus feats are poor, the DR and energy resistance are in amounts too small to much matter, and the one big thing—flight—comes far too late. The cleric’s features are flexible, powerful, slot into a whole lot of other features very well in ways that are very awkward for the favored soul to emulate, and they all come at 1st level, leaving you free to take other prestige class levels without missing out on anything.
A better option: Pathfinder’s Oracle
If you want a spontaneous divine spellcaster that actually has some unique things going on with it, you might consider back-porting the Pathfinder oracle. The class itself can literally just be used as-is in 3.5e—even its skill list includes only skills found in 3.5e, so you don’t even have to change that. Some of the curses, mysteries, and revelations might need some massaging, but there are so many options for each that it would be very easy to just ignore any that reference things 3.5e doesn’t have (and not much harder to just port things as necessary).
@Matthieu M did a nice job with his Answer, he talked about the advantage of the Favored Soul but here's the other side of the medal, the advantages of the Spontaneous Cleric. Plus the Favored Soul is different in roleplay too, this answer is merely to complete @Matthieu M's Answer and add info for players wanting to compare the two.
For some Feats Like Initiate of Astilabor you need to be a cleric, so you even get more spells like this (by RAW it works, but DM might say no because it would be unfair for the Favored Soul) see the question:Can Favored Souls choose feats that require cleric levels, such as Initiate of Astilabor? for more details.
for Prestige classes like Dracolyte who gets the Glory Domain, you gain better turn undead attempts, it's unclear if it gives it to you if you don't have turn attempts but if the DM says no, Spontaneous cleric is better. See the question:Does a Favored Soul really gains turn attempts if he chose the Glory domain from the dracolyte prestige class? for more details.
- For some prestige classes that requires domains or Turn/Rebuke attempts,
Spontaneous Cleric is better once again.
- A favored soul (see Dead Levels II) has to choose between knowledge (Arcana) or Knowledge (religion) while the cleric gets both, this can help for some prestige classes which gives more options for builds than the Favored Soul.