I have an XBox 360 so you can call me biased but I think the 360 is the way to go. The 360 has the bigger games catalogue due to the fact that it has been released for much longer. There are several top quality games such as Gears of War, Pro Evolution Soccer, Call of Duty 2 & 3 and Rainbow 6 Vegas.
There is an agruement that Sony has a monopoly on games exclusives but this is becoming much less of an issue. Games that used to be Playstation exclusives are now being brought out on both platforms due to the cost of developing these games. Why launch a game on one machine and not make money from other platforms? Also games on both will more likely be identical.
Another point is the Xbox Live service which is excellent.
If your after a next-generation DVD player then go for the Playstation 3 if your solely after a games machine I'd choose the 360.
The only major drawback of the 360 is the noise. It is fine if you are playing a game which is stored on the hard drive such as a classic arcade game downloaded from Live. However, when playing a game on a disc then the combination of the disk drive and the fan can be quite loud. You do forget about the sound and it isn't deafening by any means but it is annoying.