I own both.
Xbox 360
Pros:
Good, reliable online service
Comes with a headset
More comfortable controllers (My opinion, others may disagree)
Hundreds upon hundreds of Indie and Arcade games
Backwards compatible
Cons:
$60 a year for online
RRoD
Controllers use batteries (You can buy rechargeable ones)
Not as many good exclusives as PS3
PS3:
Pros:
Free online service
Several great exclusives
Better hardware
Simpler menu system
Cons:
Not as quality online as XBL
Fat PS3s are loud as stuff
YLoD (Happened to my 60gb PS3)
Short recharge cable compared to the 360's superlong one
honestly i'd get both in the long run