I think there is some difference in the wii version vs. the ps2 version. But I could be wrong:
Wii - wireless Les Paul guitar, ability to sync your scores on the guitarhero.com site, and online play.
PS2 - wireless Kramer guitar, unable to sync with guitarhero.com, and no online play.
The PS2 does have guitar hero 1 and 2, so once you sons finish guitar hero 3, you can always buy the older ones for more games, but I am sure they will eventually come out for the Wii, since it seems to be selling well.
It was also anounced that the Wii version would have downloadable songs later this year(February-March)
I have the Wii version, and find it a lot of fun. I enjoy playing online because I only have one guitar(they do not sell a separate guitar yet for the wii - coming at the end of January).
and playing with other people is a blast!
Hope this helps