The PS3's OS takes up a chunk of the ram, the 360 OS has its own allocated memory, to it doesn't encroach onto the system memory.
All 360 games can use custom soundtracks.
I'm sorry, but I'm not going to sit there and copy music from my PC to the ps3, that'd be a pain in the arse, and needless duplication.
Sure, the HD headset from Sony is good, but not everyone has one, so you're limited by whatever the person you're talking to has.
The specific game invite on some games fro the PS3 is none existant on 360. Every game is integrated into the live system, I can send and recieve invites from any game, and chat to friends. Even single player games.