Xbox 360:
+ Cheaper
+ Lightweight
+ Great Online Multiplayer
+ Backwards compatability
+ Easy to Flash/mod
- Manufacturer defects
PS3:
+ plays Blu-ray movies
- expensive
- shitty online multiplayer
- Expensive
- Heavy
- No backwards compatablity
- Can't mod or flash
Other:
Multiplatform games look better on the 360 most of the time. Most games are on both platforms anyways.
This is the criteria I chose to make a purchase with. The 360 is clearly a better system. The PS3 is expensive, and pretty heavy to travel around with. They both play mostly the same games, and the games are either identical in quality, or the 360 looks better. The Ps3 was specifically designed to be incredibly hard to program for so you get A) Inferior games when porting from the 360, b) longer development cycles leading to less exclusive games, and c) Game's potential is stifled, and won't see anything really good looking for several years.
All things considered if you are asking this now having not really looked into this, clearly the 360 is a better choice. If you want to watch movies in slightly better resolution on really expensive TV sets, then by all means get a PS3.