I have to go to a meeting so won't be able to respond if you do shortly but I disagree.  They are basically the same game when it comes to the guitar piece.  Yes there might be variation to difficulty and GH does have some wacky head to head modes which my friends and I hated and never played.  Other than that GH and RB guitar playing is the same thing.  Seriously GH 1 and 2 where made by Harmonix who made RB.  All activision did was add new songs and omg BOSS battles LOL.  Mind you Harmonix didn't add much to the guitar portion of RB mainly the rocking out at the end.  Again in terms of Guitar Playing they are as close to each other as they can be.  Hell I'll go out and call them clones LOL.  Seriosly list all the difference if you think they are so different and no different songs doesn't count LOL.  I think over time people will dump GH and move on RB because quite frankly it offers ALOT more.  We will see what happens but I am sticking by my guns that RB is far superior and will eventually take over it.

P.S

Your downlaoded songs WILL carry over for the next Rock Band according to Harmonix.  Is the same true for GH3?

Well being that most of the Wii owners on the site don't think much of online that could be debateable!

Ummm... Which ones would that be considering they are all exchanging friend codes for Brawl.  

Also GH3 is one of the top online games on Wii.  The fact EA left it out of RB Wii is just stupid, and while downloadable content is still peanuts on 360/PS3 overall (compared to retail revenue) there's no reason for them to leave this out either since pay to play content is already live on Wii now.  It's like they want to fail on Wii.  Worse, there's far more competition on Wii with Battle of the Bands, Ultimate Band, Samba de Amigo, Major Minor's March, etc., all competing for music game dollars.

...Late response.

The MAIN focus and idea for Rock band is the multiplayer experience while GH main focus was solely the Guitar solo experience.  So, it that way they are completely different.  Now let's just look at the Guitar portion of Rock Band and compare it to GH; aside from the difficulty the GH experience is more of a caricature of a Rock Guitarist, it's centered around the guitarist and the set list is more guitar heavy, while Rock band is more realistic, more balanced.  That is a major difference in my opinion.  I have both RB and GH3 and the guitar is a lot more fun to play in GH, it lends itself more towards pretending to be that iconic Rock guitarist WHEN PLAYING BY YOURSELF, Rock band is far better when the whole gang is there but it diminishes the experience for a guitar solo player as it enhances the band concept. 

rockband wins.

Don't know about rockband DLC, but We three kings is the awesomeness and Dream on is the best song in GH now, and they both were free.