They're both very good consoles. I'm sick of this console fan-boy crap.
I'm a 360 owner because I can't see myself saving AU$700 for a PS3.
360:
Advantages-
Huge game library
Bigger range of down loadable content including arcade games.
Disadvantages-
Masses of Hardware problems
You have to pay for online
It's Microsoft and no one likes Microsoft because they suck arse.
PS3:
Advantages-
Lots of fantastic games coming out for it (Killzone 2, infamous, God of war 3)
Good graphics
Blu-ray player (I own a proper blu-ray player, and they are very good)
Free online
1 -> 3% failure rate (which is low)
Technology is better, and it will probably live longer then the 360 (Like the PS2 which is still going strong after 9 years).
Disadvantages-
Expensive
Smaller game library (although it is growing pretty fast).
Pretty much requires a HDTV.
It's all up to personal opinion on what console to buy.