Personally, I prefer the Xbox 360 because it has more games, it has a far superior online matchmaking program, and it's cheaper. And of course, if you're into action titles like Halo 3, Gears of War, maybe a little Resident Evil 5 and Grand Theft Auto 4, you can get them on the 360 (though those last two will also be on PS3). There are also more real-time strategy titles for the 360, though I don't believe you can have much fun with an RTS if you don't have a mouse and keyboard.
Alternatively, if you are low on cash, just go with a PS2. It has a huge library of great games, and they're getting cheaper and cheaper now that the current gen consoles have been out for a while. I plan on picking up a used PS2 in the near future, just to catch up on all the games I missed while playing my Xbox and PC.