Well I have both, and I would normally say xbox 360, but IMO the 360 has a letdown that I've never experienced with playstation. IT KEEPS BREAKING!!! I have a had a playstation 2 for over 6 and a half years now, and it hasn't broke, I still play tekken 5 on it. On the other hand, my 360 lasted 3 months and got the stupid "RROD" which made the 360 lose it's appeal, there are loads of people complaining about this on xboxlive.com, I see it on the forums everyday.
If it wasn't for that, I probably would have said xbox 360, although PS3 has free online as well.