PS3 has WAY better (and a lot more) exclusives, better hardware, better capabilities in general, and a free online service. Oh, and it's way damn quieter.
360 has a better online service and a better controller.
I like them both just fine, but I honestly think the PS3 has the upper hand, and this is coming from a guy who has had a 360 since its launch year, and only got a PS3 last Christmas.
I've never played 360, but I've had a PS3 for about a year and a half. I really don't see how one is better than the other. I prefer PS3 because I've been with Sony for the past few systems. Both have their positives and negatives and their own unique traits.
Both are fine IMO, prefer online play with the 360 and I love the controller, but PS3 has awesome exclusives unmatched even by what Nintendo puts out. I tend to play more games on the 360 because I've had it longer but the PS3 gets more milage from general use like watching movies since I don't worry about it breaking as much.
Heh I still have a 20 GB Pro Model (Around 4 years old?) so I can't even install any games. No biggie though, downloadable games and the like go on my PS3 unless they're 360 exclusive.