PS3, but it depends if you want to play a bit more casual in that case get PS3. But if you fancy playing squeaky 11 year olds that just wreck whatever game your playing then get Xbox. I've got both and just don't find Xbox enjoyable anymore. Also I don't what games you play, but PS3 is a lot more customisable E.G. if you like PES then it's a lot easier to put the kits, real names etc. into the game, because you just copy them onto a USB. Whereas on Xbox you have to get download a program that allows your computer to detect the data files then transfer it blah, blah it's also really annoying for me because I have a Mac and there's not a program to do that on Mac so I have to borrow my Mum's laptop which is a pain.
Also, if you're looking into buying Portal 2 then if you get it on PS3 you get the Mac/PC version free to download via Steam. Unfortunately I haven't got it yet because it was out of stock, but I've been watching a few videos on YouTube and it looks great, so if you get PS3 I advice you get Portal 2!