Now i just got a ps3, i usually played with my friends. This is my opinions on them both, i've owned a 360 since launch though.
Firstly, 360's online is better than ps3's in every way. Party chat, faster updates, blah, blah, its just better.
PS3 has more capability for graphics, but the difference is hardly noticeable in the multi-platform games. However PS3 has more games that are HQ for it than Xbox. Along with having higher rated games for itself too. Xbox has very little console only games that are highly rated.
Honestly if you want to play with friends, Xbox.
If you want a blue-ray player, and higher quality games, ps3.
Hardcore gamer, Professional gamer (pro = getting paid pro), playing with immature kids, paying for membership after you're paying for internet and the game and the console as well as power, etc;
Get an Xbox
Love to chill, watch movies, play games hardcore or family, don't like listening to people in every room, playing with a mature community, free online play (excluding paying for internet), blu-ray (like @Empire said), HD games and movies, web browser, bluetooth capability. cheaper accessories or just plain out better compatability .
The Xbox is better for hardcore/pro gamers, kids that'd like to play with their friends, and the Kinect gaming