Xbox 360: Somewhat better online community than the PS3, much more decent and better games to play, more events and downloads, can transfer XBL accounts from one system to another, uses achievements instead of trophies, allows you to change your username once it is first set, has lots of annoying people that play online, lets you make your own customisable avatar, can not access the internet, plays videos/music, and it generally gets downloads a day or so before it gets on the PS3. It also seems to crash and have much more problems than the PS3 such as RRoD.
PS3: Has only a few actual good games on it, is much more stable than the 360 is, cant transfer PSN accounts between systems, uses trophies instead of achievements, plays videos/music, seems to have much less annoying people playing online, only has a few set amount of profile pictures to choose from for your PSN account, does not allow you to change your username once it is set, can play blu-ray DVDs, and can access the internet.