It really depends on many factors, such as world size, number of opponents, specific opponents, how much mana there is, etc. You wouldn't find Expansionist too useful with a magic wielder who can access all spellbooks, and a bonus to summoning wouldn't help someone who's real benefit lies in churning cheap melee troops. Again, in a small world, magic users are at a disadvantage unless you ramp up mana, but in huge worlds, they can become pretty damn formidable.
