Both are great consoles, and I believe that comparing them is like comparing Soccer and Baseball. They both have their own merits, pros and cons.
But let me compare the online services for you.
While you have to pay to play online with an Xbox, the online connectivity is great and there are not much problems. 8$ or so a month is worth it, in my opinion.
On the other hand, you can play online with a Ps3 for free. But I really did not like playing online with my ps3, because it give me a lot of issues and lag. Maybe it would have been different if I had better internet, but my xbox gave me better services even with the same internet provider.
I guess that the price of playing online IS proportionate to the quality of online-gaming.
If your internet is average, I would recommend the xbox 360 for playing online.
However, if you have awesome internet, maybe the Ps3 will do you good.