Both are great!
Xbox
Pros: Over 14 Million players online, Many Great Games, More features such as downloadable netflix, ESPN, And hulu is coming in January! Accesories are generally cheaper, such as a mic for only $20 (they suck but they are cheap)
Cons: Disks and only hold 8GB so games cannot live up to their full potential. Xbox live is not free, It is $60 a year, $29.99 for 3 months, and $10 for one month, How ever if you have 2 or more accounts you can get a family pack for $100 and it will supply Xobx live for up to 4 Accounts. Does not Have Blu-Ray or HDDVD built in (HDDVD is an extra addon) 3D is only on certain games. Built in WiFi for only the Xbox 360s monels
PS3 (I personally don't like the PS3, but I will try to give a good overview!)
Pros: Free online, Disks can hold 30+GB so games can live up to their full potential. Supports 3D on the XMB. More universial as it uses blutooth to connect its devices. Can use Netflix with the required DVD. Built in WIFI in all models, you do not require the new SLim model. Disks are harder to scratch, but can get ruined if they get wet for too long.
Cons: Less people online, Online is a bit laggier than xbox. Games cost slightly more. Not as many features as Xbox.
If online is your main concern get the Xbox, $60 a year is not alot considering everythign you get, netflix, ESPN, Hulu, Last.fm, 14Million+ Players.