I own a 360. Invested in one about 6 months ago because I got a really good deal and if I wanted similar with the PS3 I'd have to pay £100 more.
If you love online gaming, you really have to go with the 360. Yes, there is an added price but I'd gladly pay a small fee if it meant a superior experience.
However if you aren't too bothered about online, then the Blu-Ray player built into the PS3 will probably sway you (as long as you have a HDTV to play it on).
Technically the PS3 is the more powerful machine, but at the moment you won't see much difference in quality because the developers have a harder time getting the most out of the console. In a few years that will likely change, but at the moment there isn't much of a difference in graphics between the two consoles, in my opinion.
All the British MMA guys