XBOX Live has many times more content for download than the PSN.
XBOX Live has background downloading for ALL content, and even allows downloading when the console is off.
XBOX Live features chatting, video messages, invites, regardless of what your friend is playing.
XBOX Live tells you who's online, what they're playing, what level they're playing on, etc.
XBOX Live is broken down into communities, (recreational, underground, etc.) so that a casual player doesn't go head to head with a video game grand champion unless they choose to.
XBOX Live has scheduled family nights, girl gaming nights, tournament nights, and other event nights (www.xbox.com/primetime for more details).
XBOX Live has downloadble stratagey guides, documentaries, and other non-gaming, but gamer culture related content available for download, (and a handfull of quality free games on XBLA).
It has a ton of other features, such as PC interaction, instant messaging, and such, but I think that the PS3 offers many of those features.
