I'm getting set to get back into DMing for the first time in a couple years. One of the people I have interested in participating in my game wants to be a Magus for a class. Back when I DMed, I never worked with Maguses and the closest thing I ever worked with was a Wizard. What are the differences and similarities? Are they relatively similar, or completely night and day? As a DM, how might I prepare the game differently for a Magus as opposed to a Wizard?
Wizards have the full 9 levels of spells, Magi only have 6
The Magus class has its own spell list with a few unique spells. The Magus list focuses mainly on combat-relevant spells that prioritize damage and enhance mobility. Their out-of-combat utility is limited
The Magus is as much a martial class as it is a casting class, if not more. The Magus's signature ability, Spell Combat, works with full-attacks. Additionally, their class bonus feats encompass both the Combat and Metamagic Feat lists. You're more likely to see a Magus 'behave like a Fighter' than like a Wizard.
They are similar in that:
They have a number of spells in common on their spell lists
They are both INT-based arcane, prepared casters requiring a spell book
Most archetypes of Magus can get a familiar
So what should you take into account when building a campaign including a Magus rather than a Wizard?
Combat Range: The Magus will almost always be fighting in melee range and use their spells for limited long-range combat. (Unless the player chooses an archetype that focuses on ranged combat, like the Myrmidarch or Eldritch Archer.) When building maps, consider that forcing a Magus into tighter quarters is probably what they'd prefer, to make use of their Spellstrike class feature and use touch spells in melee combat. Wider open maps put the Magus at more of a disadvantage at lower levels because of the limited amount of ranged spells they get.
Combat Style: Like the Fighter, the Magus works best with full-attacks, even though they are only a 3/4 BAB class. Spell Combat encourages single-target full-attacks to quickly take enemies down. Single-combat or encounters with few large enemies work in the Magus's favour, while fights with many smaller foes can get tricky and force a magus to burn through more spells. Bear in mind that the Magus list includes strong AoE spells at level 3 and onward (and a few weaker ones at level 2) so avoid swarms and grouping up your goons.
Utility Spells: (Or lack thereof) Compared to a Wizard, a Magus is much less likely to have spells to solve out-of-combat situations. The Magus spell list is very combat-oriented, making it light on utility spells. Solving a problem with Divination or clever use of Illusions is not really an option for the Magus. Instead, try encouraging your Magus to make use of their wide variety of Skills out of combat.
Action Economy: Consider the Magus to be a kind of arcane equivalent to the Paladin. Rather than Lay on Hands and Smite Evil, the Magus gets Spell Combat and a number of Arcana to broaden their combat capability. Both classes highly value their action economy and make liberal use of swift action abilities. If your Magus asks for help picking Arcana when leveling up, encourage them to consider their action economy and remind them they have a limited number of swift actions per round and will have to prioritize some abilities over others.
So to summarize: The Magus tends to lean heavily in the martial direction, and its similarities with the Wizard are mostly general and superficial. Hopefully that helps some!