Having owning both systems I can say this. The 360 is a lot more fun online because everyone has microphone, and uses it. It also has achievement and gamerscore which I like. The only thing is I'm not a fan of the controller, I'm always scared when I turn my console on, and a lot of people online are d*cks. I don't consider having to pay for LIVE is a bad thing, since its $50 for a year, which works out to 12 cents a day, which is hardly anything.
With PS3, since not everyone has a mic, it's a lot more quiet and theres less communication, which isn't good for strategy/teamwork games. The GUI is a lot more simpler and easy for anyone to understand. And with the release of HOME coming very soon, the PS3 will also have in-game XMB and a new innovative way to interact with your friends and people around the world with your avatar character.
In the long run it depends on what games you like, because even if a system is uber-great, if you don't like any of the games on it then you just wasted $400 for a pretty interface.
For me, I enjoy my PS3 more since I'm not the hugest FPS fan.