Xbox360 mainly because its more popular and most people i know own one. My PS3 gets all the Sony games/exculsives. Out of the two i find the xbox is better for LIVE enabled games and more of a community is going on but have IMHO found the Sony inhouse stuff/exculsives to feel more polished than the best of what the 360 can churn out. I feel that the PS3 has a lot of catching up to do as a community based console which hopefully HOME may achieve but it just feels as if this side of things are rather "just tacked on" rather than the seemless nature of the 360. Also prefer the 360 controller which is more suited to FPS and driving and better triggers, again hopefully the new PS3rumble controller can sort that out. Also find the 360 a better media device with Windows Media Center and a PC. Don't like the XMB of the PS3 either, looks naff even with the new themes. Blu-RAy or HD-DVD couldn't care less as i never use either or don't own either formats for film watching although seeing the PS3 has it built in i'm more likely to buy a Blu-Ray movie and was impresssed with the free Casino Royal that i got 1st time round. And then achievements, love them or hate them they do add to gameplay expericence, its nice to see it popup on screen.
2008 is going to be the PS3 year, either it will succeed or fail and right now i'm still not 100% certain where Sony is going with the console although slowly things are getting better, its just this time Sony do have a major rival console breathing down their neck which they ain't used to and atm IMHO it is struggling to compete, the coolness of the brand "Playstation" seems to be not so cool and the 360 seems to be the "grunge" console, lol that everyone loves or hates or "loves 2 hate"
anyway thats what i think