I don't own a 360, but in order for you to play online you need to pay a yearly fee of $50 which I think is pretty ridiculous. The PSN is completely free, and you'll never have to pay a dime. You will find more people on the 360, though but I lurk on SRK a lot, and it seems more of the heavy hitters are on Playstation 3.
As far as the communities go, the 360 has a lot more 11 year old kids playing online than the ps3. This leads to a lot of ragequits when you are about to win a match.
Also if you get a 360, you have a very great chance of having to send it to Microsoft due to the infamous Red Ring of Death that causes your console to stop working completely which is pretty humorous to say the least. I've had my ps3 for 3 years now, and I've never had a single issue with it. I know someone who has had to send their 360 in in total of SEVEN TIMES because of the red ring.
The PS3 looks more sleek in design, while the 360 looks like a flimsy toy. In my opinion, Xbox is a bane on the gaming market.
Good hunting!