Well the Xbox version will allow you to purchase the exclusive extra missions that Rockstar are only releasing for the Xbox. Not sure how they compare graphically though as I have neither machine :-(
To be perfectly frank I would go for the XBox version. The graphics look crisper, the online is better and as Booldawg says you will be getting the episodic content (albeit at a cost).
Before the PS3 fanboys chip in, I have it on the PS3 and it's not as good as on my friends XBox.
The other thing to consider is if you want to play online with mates, what format do they have it in?