I own a PS3 and I would recommend getting an Xbox if you're basing your choice directly on this game. More people playing online, and the 360 version will get DLC before the PS3. The PS3 will eventually get DLC (it's inevitable), but not right out the door like the 360 version, and it may not get the same DLC as the 360 version due to exclusivity agreements (probably timed exclusive like GTA3 on PS2).
But, at the risk of sounding like a PS3 fanboy, I don't really care about DLC. They're just squeezing extra money out of you for stuff that should have been in the game in the first place--the DLC is coming out parallel to the game. This means they've been done with the content for a while... they could have easily made it a part of the game. Not to mention the fact that the game is absolutely massive and has enough stuff to occupy you for a long time without having to download additional content. You're not really missing out on much regardless of which console you choose.
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