I just bought a PS3 after debating the two and ultimately I looked at it like this.
1) I need to rationalize the purchase with my wife and the PS3 comes with a Blu Ray player.
2) Most of my friends don't play games online, so I'm going to be playing random strangers on either system.
3) Most of the 360 exclusives are FPS's which make me sea sick.
4) Little Big Planet and of all the wierd PSN exclusives.
5) Price. Though the arcade recently dropped to 200, for my current needs it was still more expensive after purchasing what the PS3 80 gig already comes with at $399. See below for the math:
360 Arcade - 200
Online Subscription - 50
WiFi Adapter - 100
Hard Drive - 100 (This is a rough estimation as the 20g is $70 and the 120g is $140)
HD DVD - 50 (outdated yes, but it is the only current option for true HD movies)
Grand Total = $500
Note - Neither system comes w/ an HDMI cable but they can be purchased online for $3.
6) Longevity. Given the fact that the 360 did come out a year earlier and the PS3 is just now gaining momentum, I have a feeling we'll continue to see many new releases which push the hardware over the next few years or so. MS on the other hand, may release a new console in late 09/early 10.
PS - If you want to buy new and save 100 bux, here's an offer I used that worked great. I ordered the system on Tues and had it by Fri w/ their normal shipping.
http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644489278