64 gig 4S.
I'm upgrading, because I'm a gamer. And the newest tech is what I need to experience fully what the app store has to offer. I know Real Racing 3 will be tailor-made for the 5, as well as stuff like Infinity Blade: Dungeons.
Also, that new bit of screen real estate will mean my fingers have more room to breathe.
The new phone is still no Vita alternative, but it'll do in a pinch... or on the bus, or on the toilet, etc.