Assuming you had a PC of current gaming spec and also owned the current consoles (PS3, 360), if a game is released cross all platforms which version would you normally buy? And why?

Does it come down to a preference for a particular platform? Or do you do some research first to see which version is the best?.. f.ex. if a game is developed on the 360 then ported to other platforms it often means the 360 version is generally the best one.