The only problem with the PS3 is the less than great selection of titles. :/ It's got a lot of the same games as the Xbox 360, but way less exclusives. It's got MGS4, Killzone 2, and LBP, which compared to Xbox 360's list isn't that great.
Uncharted, Uncharted 2, God of War III, WipEout HD, Valkyrie Chronicles, Ratchet and Clank, Motorstorm, 1&2, GT5, Siren:Blood Curse, heavy Rain, SingStar etc would all like to say hi.
Bearing in mind I only named exclusives that I have/have had/will buy and would recommend. I deliberately missed out games like Haze and L.A.I.R on the fact that Haze is just Halo but somewhat worse (A great feat I know) and L.A.I.R Should of being titled F.A.I.L.
360 has what, Gears 2 and Fable 2? They're the only two decent exclusives I can think of. I guess it has Lips, but face it, no one wants an inferior clone of SingStar.
XBLMarket Place does have Castle Crashers tho <3
Anyway OP, I'll be honest here with you. Buy a 360 if you like shooters or racing games or being abused online, if you want something with more in depth/casual/platforming/story based games go for PS3. 360's online is apparently better, however all I;ve ever had from it is a bunch of racist 12 year olds screaming. It also costs you £40 for 12 months. Playstation has near enough the same amount of players online now, but the online is free. On 360 you get
miis avatars, on PS3 you get
Second Life Home.
Both have Call of Duty 4, both have Burnout paradise, both have Resi 5, both have FF13, both have Bioshock etc. So really, you won't be missing out on the best titles whatever way you go, bar for games like MGS4 or gears 2.