If you want a game for online gaming i would suggest the Xbox 360 as it has more players online and is just generally the chose for online gamers.
If you game alot with family and friends at the same house then chose the wii.
A PS3 is not really that great but is has got some nice features to it like internet browsing, blu-ray player and good PS3 exclusives.
As far as the Wii goes in november there is a add on coming to Xbox in about a months time called Kinect which works the same as the Wii but with no controllers and also with the benefit of the Xboxs high end graphics.
My preference is Xbox 360 as it is just generally a nicer, easier and more enjoyable gaming expieriance