Well I own both, and both have their own use: (this list is my personal observations on the consoles)
Xbox360
Pro's:
+good online multilayer service
+Controller is suited for FPS's
+Cheaper
+Includes headset
Con's:
-Need to pay for online play
-Netflix needs Xbox Live Gold
PS3
Pro's:
+Can play blue-ray disc's
+Good exclusives
+Free online play
+does not need extra fees for Netflix
+has a web browser
Con's:
-Does not come with a headset
-You need to buy an HDMI/Component cable with it
The headset thing is important, since the PS3 does not come with one most people don't have it. You could probably categorize it as both a pro and a con, depending on what type of people you want to talk with. If you want to use Netflix on your Xbox you need a Gold ubscription, on the PS3 you only have to pay for Netflix itself. However the PS3 consts more, well at least it did for me. Like others have pointed out the only exclusives on the Xbox is Gears, Halo, and Fable.
The PS3 has Ratchet and Clank, Uncharted, Demons Souls, God of War, LittleBigPlanet, and Killzone.
In the end I would recommend the Xbox360 if you play mainly online, and the PS3 if you mainly play singleplayer. It also depends on what console you friends have if you want to play online.
Ps, The Wii would be crap, but there is a shining beacon of light in the pile of shovel ware: Monster Hunter 3 Tri. If you own a Wii without this game, your doing it wrong. You also lost the game.