X gets hundreds of new busters, new armor, a lil dog sidekick, new recruits practically with every game and can copy the attacks of his fallen foes.
Zero gets a few new sword abilities (some of which, at times, don't seem to do anything), no new look, no sidekick, and was quite literally a samurai in Protoman's template.
Quote Originally Posted by Walter
That said, Zero is extremely overrated.