Tough one, I have both
Better gfx on exclusive titles and their exclusives are better as they are not stuck to DX9 gfx, I did play Uncharted, Heavy rain while I didn't play any of the 360 exclusives like Halo and Gears, a bit too juvenile for my taste.
Better Media Player
Upgradeable hard drive
Quiet! but I assume the slim fixes that
Compatible with PC racing wheels if you are into racing games with a wheel.
The online store is utter crap, so digital distribution of AAA titles is non existent.
Better overall interface although the recent upgrade is pushing the advertising too far.
I prefer the larger shape controller
Better performance on games that are DX based which seems to be the majority.
Online store is OK but rip off pricing so you would still visit the store, yes even for games that are 2+ years old. So compared to the PS3 it's basically shit just for a different reason.
I don't play multiplayer games at all on consoles but no doubt the 360 is the better platform there albeit not free.
Given what I know today, I woud buy a PS3 tbh, but my reasons may not match yours though.
I hardly play console games for starters and if the game is on PC I'll get it there, most multiplatform are on PC as well and that leaves the exclusives which for me are more appealing on PS3 as mentioned earlier.
This choice would be totally different if it wasn't for my PC though, if I did not play on PC I would get a 360.
Another reason is that it's pretty much confirmed that we will be getting the 360 sequel announced at E3 next year, ps4 is a little bit less clear.
Not helping am I