ARCADDER has a pretty solid argument for the PS3.
Being an XBOX owner, LIVE seems to have a more reliable online gaming experiance from what I have personally seen. Sure you have to purchase more stuff for the XBOX, but that is why the consol came out about 100-200 dollars cheaper than its Sony competitor.
XBOX has it's own set of exclusive titles, like Halo as mentioned above, but also more like Mass Effect(which is rumored to go to PS3 anyway, so it doesn't really count), Gears of War, Crackdown(which was a decent game), Lost Odyssey(fantastic RPG), and a few others I can't think of at the moment. Both systems have their own plethra of features on the dashboards as well.
XBOX doesn't have the best rep with having good craftmanship on their systems, with the entire RRoD fiasco, but with the new "slim" versions(what I currently own), I have not had heating issues sense I bought it when it was launched a few years ago.
So really, you will end up paying around the same for both if you think about it. It really depends on what you want to play and get out of it. I would say dig through a bunch of games and figure out what you want to play from the exclusives, and choose that specific one.
Or go PC ^_^ lol