Well... seeing as I have both, my opinion will probably be quite constructive.
I avoid being a fanboy type!
Xbox 360
Pros
Simple yet rewarding achievements system
Strong titles such as Gears of War and Halo
Well established LIVE service almost 6 years old
Simple user interface
Comfortable controller
Relatively Cheap
Cons
Prone to overheating
Microsoft are a US company
(if you aren't a US resident, its tough to get any help!)
Online setup costs can be quite expensive
PS3
Pros
Free to setup online (no adapter or online service costs)
Most powerful graphics processer of all systems
Sixaxis controller
Strong titles such as Metal Gear Solid
Soon to be released Playstation Home
Classy looking machine which feels well built
Region free
Built in harddrive
Cons
Not a very well established online service
Quite expensive
Uncomfortable controller design
Few games to play
With that said, I am mainly an Xbox360 player... but the year before the PS3 came out, I was hooked on the first next gen offering and it was hard to turn back to Sony.
Im sure youll make the right decision, they are both great consoles!