Didn't the first Wii Samba De Amigo sell about 14 copies?
It was recently available as an Amazon Deal of the Day for $7, if that tells you anything.
My preference for the Rock Band franchise is based on several factors. I consider the core experience that the games provide as basically the same, so I haven't hopped on either bandwagon. I also have to shake my head at the fanboys who go on about how "playing Rock Band on Expert is like playing Guitar Hero on Hard". All I can really say to that is...congratulations for having enough time on your hands to care about stuff like that.
What I like about Rock Band is that their DLC is always stuff I don't expect. Guitar Hero DLC seems to be deliberately mainstream and seldom strays off the beaten path (as far as I'm concerned). Rock Band, on the other hand, has weekly DLC that often makes me say, "Wow, it's cool that they released that track, but who the heck is going to buy it? That band is so obscure!" As a music fan I really appreciate that, because it tells me that they look at DLC as more than just a recurring source of revenue. I could be way off-base, but it seems that they actually give a crap about exposing people to new types of music.
Their extensions of the franchise also seem to be done in measured steps, whereas Guitar Hero wants to be everywhere, all the time. Aerosmith? Metallica? Band Hero? DJ Hero? Modern Hits? 80's? On Tour? World Tour? I can't make sense of it any more. They're diluting their brand, and turning people like Caterkiller off to the entire genre. Sure, Rock Band is getting a little more whorish - Rock Band Beatles, Rock Band Pearl Jam, Lego Rock Band, Unplugged - but these releases seem like logical directions in which to take the series rather than blatant rehashes or cash-ins. The Beatles game will have a ton of fan-service extras, and will surely blow away the GH Aerosmith/Metallica games in that department. I'm sure the Pearl Jam game will be handled the same way (for starters, they had fans vote on which live tracks they wanted on the track list).
Also, like somebody mentioned, the fact that Harmonix allowed you to import music from RB1 into RB2 was huge. It showed me, as a customer, that their first priority wasn't to gouge me for every penny they could. Meanwhile, Guitar Hero is over there using the GH III engine for a mediocre Aerosmith game that really didn't need to be made.
I'll never write off a Guitar Hero game, but it'll have to be considered to be flawless (i.e. it can't be missing stuff like the ability to save people in multiplayer) in order for me to pick it up. Rock Band got my money first, and I'm pretty much entrenched at this point.