There isn't a versus there. Apple v Oranges.
What do you want to use it for?
Who is going to be playing it?
If *you* are going to be playing it, I presume you should see what games you like and are available. Wii has an entirely different library/genre of games than XBox360/PS3.
Young kids and/or yourself wasting a very small amount of free time, get a Wii. Games are fun, no learning curve, no huge time investment required. Wii Sports is fun, as is the whole Mario genre.
Teenagers, and/or yourself wasting hours, get a XBox360 or PS3. Graphics are great, games take hours to learn and hours upon hours to complete.
As for XBox 360 versus PS3, each have a few exclusive titles (XBox 360 has more, like GTA IV); PS3 has firmware updatable Blue-Ray player.
I will have all 3 systems.
Wii is played by my 5 year old, and is the only thing I play (given my maximum time investment isn't more than 30 minutes an evening). The cousins play it when visiting, it's great fun. Connected to a 21" Tube in the game room, would like to buy another 32" LCD for it.
XBox 360 is played by my 12 and 15 year old. They waste time "beating" games; I could never sit in a chair that long unless I am working. Connected to a 32" LCD wallmounted in the game room.
PS3 is on order to serve as a Blue Ray player in the living room. Kids can play it only before adults wake up. Connected to 50" Plasma.