This is a good article comparing the two on C|Net.  The article market research firm NPD as providing the following numbers for November sales:

• Xbox 360:  511,000
• Wii:  476,000
• PS3:  195,500

Obviously it’s a lot easier to get an XBox 360, but outselling the PS3 by a lot and a one year headstart are a significant advantage.  What did they think about the quality:

The problem is that I didn’t agree. I looked at Gears of War and it looked great. But so did Resistance: Fall of Man on the PS3. To my eyes, which are admittedly not trained to see the minute differences that can show up on an HDTV, the two war games seemed about equal in quality.

And his conculsion:

I just hope these guys realize that for all the differences, the similarities in the performance of these two boxes are remarkable.

Given that the Xbox 360 has been out for a year longer, is substantially cheaper ($200 less comparing retail prices of premium versions), readily available, and has sold at least 5-6 million more consoles…. this seems to me a pretty strong endorsement.  If I’m Sony with my new technological marvel, the PS3, that I’m trying to sell on the basis of its serious performance enhancements over anything else – this is not a good thing to hear.