The one downside to Expert archetype for a Magus-like character is that you can't start with ASC at 1.1 (unless you go Primal or Psionic). If that doesn't bother you then Expert may be a good option.
Basically, the tradeoffs are:
- Arcane
- + great casting and lots of arcane talents
- - melee skill will fall a little behind and lacking martial talents
- Martial
- + great melee and lots of martial talents
- - casting skill will fall a little behind and lacking arcane talents
- Expert
- + good at both casting and melee skills
- - greatly lacking in combat and arcane talents (in comparison)