There's TONS of info out there on both systems.
360 will do 1080i and 1080p, but if you don't have 1080p tv, then your best bet is to run games at 720p as it seems to be a bit better for high motion on the games.
I would pick 360 at this time because they have a normal game selection already and are on to the 2nd generation of games to better use the hardware. As opposed to the PS3 where it's simply anticipation. Price is also a factor since the 360 is much cheaper than the 800 or more you'll pay for a base PS3 at this time. And of course the bugs. The bugs will exist in almost any system. MS has finally ironed most of them out. If only EA did the same with their servers so I could play online without having to shove a butt plug up my ass every other time. So at first you'll have to live with PS3 bugs, and no doubt, in a year the console will be on version 1.1 or 1.2 with some slight modifications. I think the PS3 is a great system, but just not right now. Oh and of course Xbox Live is the way to go for online play.
As for backwards compatibility, just leave your PS2 hooked up or your original Xbox and you have 100% backwards compatibility. Both systems have native 1080p, so not an issue there. The one final thing I'm not jiggy with is the bluray being included with the PS3. I don't want to pay $600 for a system with Bluray right now since it's really not clear who's going to win the war yet. PS3's are selling like crazy, however HD DVD players are clearly cheaper and outselling Bluray...who knows who will win.