360 looks like it will have more longevity than the ps3 version simply because of the live element and extra content... that is of course if the hardware holds up to the game. tough call really
The game has been developed for the 360 this time round and is being ported across to the PS3, ergo buy it for the 360.
Rockstar have confirmed that there will be exclusive downloadable content for the 360 version as well and as Darkmajes says, the online side of things is considerably better with Xbox Live than Sony's appalling attempt at doing the same thing.
However, if you haven't pre-ordered the game, good luck finding a copy
We have bought it on XBOX 360. we queued up at 5:30 for tesco to open at 6 and got a copy. we are lucky that our local tesco is not too busy, so I will be banished from the lounge this evening while my boyfriend plays this game until god knows what time!!
You were lucky, I was our at midnight for mine only to find about a hundred people queuing at Tescos. Got told they only had 70 copies so joined a mass trundle down the motorway to Asda where I got lucky and got the last copy.