Well, there's pros and cons for both of them, and all in all I believe they are quite similar machines.
Graphics can't really be used as an argument for either, they're very close. The PS3 exclusives are slightly prettier than the 360's, but multiplatform titles tend to perform better on the Xbox.
Price.. The Xbox is cheaper to buy, but you have to pay to play online. And btw, don't know how it's like in that states, but around here you're a sucker if you actually pay the full Xbox Live prices - buying 12 month codes on various websites are way cheaper.
Controllers.. If you ask me, the 360 controller in superior, but that's very subjective, of course. Though I have managed to "convert" some of my PS loving friends
But looking further than the standard controllers (especially when you're into racing), wheels for the Xbox is pathetic. There are none right now that are worth mentioning. Especially if you want Force Feedback, which you do. Fanatec is bringing out a new one I believe, and that's you're best shot.. But that's pretty expensive. The PS3 has a far better selection on that front.
Games.. Again, very subjective. When it comes to racing games, you've essentially got Forza Vs. Gran turismo, and that's a whole other discussion all together
GT5 is jammed full of content, but at the end of the day, both are very good games. The rest of the good racing games are multiplatform as far as I know :b
But in the end, I believe this is the point that should decide it for you. Which games do you want to play? Forza or GT? Halo or Killzone? It is _the_ largest difference between the two, and arguing all the other points gets you nowhere, and is pointless if you ask me.