I have ps3/360/wii. Obviously the wii is only party games and stuff so I dont really bother with that for console games.
When I go to buy a game that's on both consoles, I consider who I am going to play the online stuff with, so depending on what style of game and what console those friends have. I dont really consider the graphics etc, as previously said, it's not that different over both consoles.
if there is heaps of online stuff I tend to go for the 360 cos I think xbox live is a fair bit better than the the online psn network.