Two good chip shooters. Pretty tough call.
I think AZ has better graphics, though AB's are great. But I think AB has better tunes, though AZ's are great.
So they're pretty even aesthetically, but AB has smoother gameplay. AB is also longer and tougher and calls for more strategy. And while AZ's shooting action is pretty straightforward, AB throws in things like the speed tunnels and gravity effects.
AZ has more personality, with the wacky allies and all, but I'll take AB's challenge over AZ's charisma.