I've always liked Harmonix wayyyy more than I liked Activision. Where Harmonix is a music themed videogame company, with actual musicians employed there; Activision is a massive generic producer who gave the position of producing a music game to a SKATING game group. Yeah.
To me there has always been a 'soul' component missing from Activision. Instead of looking at their product and saying 'how can we improve our overall quality of game?' they say 'how can we make more money and possibly knock off Harmonix at the same time?'
Plus they completely ran the GH franchise into the ground. They even stated this in a random article on kotaku (I wish I could find it to link it here, sorry!). It went along the lines of 'We know we make excessive amounts of money from each game anyway, so we intend to release 11 games over the next year'. Had the games been improvements on their predecessors, sure, I'm all for it, but the ones they did release... weren't. In my opinion anyway.
Honestly, a lot of this is just opinion from me, some people like GH better than RB for whatever reason. Personally, I just liked to get the gme that was made my people who love music, rather than the peole who love money