If you haven't used both phones, I'd steer clear of comparisons... The iphone is much slicker than the instinct. I'm not saying better, or more feature-rich, but slicker. The Instinct doesn't even know if it's in landscape mode, you have to tell it to rotate the screen (and it'll do it for you in some modes, but it's mode dependant nonetheless).
It seems that for the "cool" crowd the iphone is the shit, and for bussiness folks it'll be Blackberry... Instinct is somewhere inbetween.