Ultimately it's more to do with what you want out of your gaming experience.
The 360 is a general good all rounder and it's where the vast majority of FPS/TPS games, as well as their player base dwells, see Halo, Gears, CoD etc. (Aware that the latter is an all platform game, but the numbers are highest on 360)
The multiplayer support on 360
can be considered to be of better quality given the fact that it's a paid service, but it's only really reflected in the general stability of the system. Don't think that because it's paid, you'll be getting away from all the obnoxious kiddies.
The PS3 on the other hand is good for some of the larger triple A exclusive releases and generally caters towards gamers who prefer single player experiences more. Sure, the online works fine but some of the best games out there on PS3 are much more geared towards single player.
I will say one thing though, PS3 games tend to have some more...
Technical issues compared to 360 games given that most PS3/360 multiplat games tend to be direct ports from the 360, again, bigger player base = hello largest portion of your market = let's make sure the game works for them and bugger anybody else.
I personally own a Ps3 for the decent exclusives that actually matter, see; Dark Souls, and use my PC for everything else.
Go for what your friends have, or what you'd prefer out of your system.
Though I'd recommend holding onto your money and building yourself a high spec gaming PC. They're not as terribly expensive to build as people make out, the components for a high end machine can set you back somewhere in the region of £600, a bit more if we're talking monitor, keyboard, mouse etc and it's basically pricier lego. The whole "I'll have to upgrade in a year to keep up with modern games!" argument is complete bull as well as with the right components and a minor bit of technical know how, you can keep your rig relevant for quite some time. Friend of mine built his about 6 years back on the cheap and could still pull medium/high settings on games out today. It's only BF3 that's made him upgrade.
Ultimately though, it's your money, just a suggestion that may be worth considering.
