360 Pros:
-Xbox live is superior to PSN
-Can choose the cheaper arcade if you're not concerned about a hard drive (and if you're willing, you can enable select drives such as Western Digital BEVT to function as a 360 hd, for less than an "elite" would cost)
-360 controller is compatible with Windows
-In addition, it seems that many people prefer the 360 controller to the dualshock
-For your personally, you can grab Forza 3 as an alternative to GT5
360 Cons:
-You must pay a monthly subscription for Live, MSRP $60/year though you can sometimes find deals for as low as $30/year
-In addition, a Live subscription is required for Netflix streaming
-Some people say that there is a larger amount of immature kids on live, when compared to PSN
-If you install a custom hard drive you run the risk of being banned from live (so you have to decide if the savings are worth the risk...though I'm not sure how much of a risk it is)
PS3 Pros:
-More exclusives than the 360, especially if you have a PC capable of gaming
-PSN is free
-Blu-ray drive
-You can always upgrade the hard drive to a larger size without fear of being banned from PSN
-For you personally, has GT5
-You can use the Dualshock with Windows, though you'll have to download some software to run it, and will probably need a bluetooth receiver
PS3 Cons:
-The general consensus seems to be that PSN is inferior to Live
-Higher base cost; no barebones option at a lower cost
-It seems (this is just an observation) that many multi-platform games are better optimized for the 360
This is just what I've gathered, as I only own a ps3. I'm sure you can fill in the rest based on what others have said. Decide what's right for you. I'll echo that if you want to play with friends, get the system they have. I also agree with mmntech's assessment; if you have a gaming computer go for the ps3.