I have both and use my 360 a lot more. Most of my friends have 360 and thus all of my multiplayer games like COD and such are on 360. Though if it was not for that I woudl not own a 360 quite honestly. I am not into halo and these days exclusives dont last that way too long like Mass Effect....
I find that the PS3 looks better, SOUNDS better (can actually handle 7.1 unlike the 360 which is still at 5.1 though I know 99% of people do not have 7.1 like me), online is free (and has greatly improved recently) ect. It is my main movie player again due to the ability to read 7.1 when available.
If more of my friends had a PS3 then I would just use that. I do not notice much difference between the online environments since I use them to play games and not hit up the stores and all that. 360 charges me though and starting next month I believe they are raising the price which sucks a bit more...
I do find it funny though that my SONY PS3 interacts with my computer better to stream music, videos, and such then my MICROSOFT 360. lol