You really can't go wrong with either one. The consoles are so similar it really depends on personal preference.
I've always stuck with Sony through Playstation 1 and Playstation 2, but for this generation I have to say that I personally like the Xbox 360 a bit better. I'm a big fan of online gaming, and that's where Xbox 360 is best. Sure PS3 has free online, but in my opinion Xbox Live is worth every penny. I also like the 360 controller. It's as perfect as you can get with a game controller.
Playstation 3 does have better exclusives though. I've never been a Halo fan for some reason, and I can't really think of a 360 exclusive that's a must-have. And I admit I did get the dreaded Red Ring of Death on my 360 last year, after 3 years of running perfectly. But Microsoft fixed it for free, their support is pretty good when it comes to the RROD.
I do want to get a PS3 eventually. I've been dying to play games like Metal Gear Solid 4, and Uncharted. But the industry is moving towards fewer exclusive titles, with titles like Final Fantasy now available for both consoles.