I just went through almost the entire games lists on VGC for these (I missed the last 3 pages of 360 games) And here is a list of games I would consider buying if I had either console... bolded are the ones I would probably buy, (and would likely end up with a smattering of the non-bolded)
PS3
Grand theft Auto 4
Call of Duty 4
Gran Turismo 5 (when it released)
Little Big Planet
Burnout Paradise
Mirror's Edge (I don't know much about it, but it looks interesting)
Formula 1 Championship edition
PSN
Probably a few pixeljunk games and some of the other unusual things like flower and flOw etc... would likely end up with more PSN gams than full PS3 games.
360
Grand Theft Auto 4
Call of Duty 4
Mass Effect
Forza Motorsport 2
Burnout Paradise
Viva Pinata
XBL
1 or 2 like Geometry Wars (If I still have my Wii, probably the second one as I have RE1 already) Likely Braid too
Edit. GRiD is another possibility on both consoles.
