I have several Guitar Hero games and Rock band 1 & 2, though I have them for the XBox 360 so I can't comment on the Wii instruments.
My family prefers Rock Band (and Rock Band 2) over the Guitar Hero games. For us, the deciding factor is the track lists - we prefer the songs on the Rock Band games. If you think your kids might end up playing at some point be warned that both games have lyrics that aren't completely family friendly. (Some songs have words that wouldn't make it through the Dis filters.) Rock Band has two new games coming out soon, Beatles Rock Band and
Lego Rock Band, that should have more family-friendly lyrics than the games which are already out. Since you can use the same instruments on all the Rock Band games you could pick up one of those later to use with any instruments you already have. There's also quite a bit of downloadable contect available for Rock Band, including a few songs from Spongebob Squarepants.
Guitar Hero World Tour and Guitar Hero 5, which comes out today, have a mode that allows the player to write and play his own music. That isn't something we enjoy but if your husband would enjoy it then that's something to consider since there isn't a similar mode on Rock Band.
Guitar Hero 5 is the first game to allow you to have multiple drummers, lead guitars, bass guitars or vocalists in your band. The other games allow only one of each to play at a time. (Beatles Rock Band will also allow multiple vocalists, but they will be singing different parts. You won't be able to have multiple people playing the same instrument or doing lead vocals on that game.)
The other gameplay is very similar on the two franchises. On the 360 you can use the instruments from either game to play both games, but I don't think you can do that on the Wii.
Anyway, my suggestion if you are torn about which to get would be to look up the track lists for the different games and see which one has more songs that would appeal to your husband, since the game is the most fun when you're playing songs you enjoy listening to.