Rush did praise Ron ... sometimes.
My guess is that he is willing to praise Rand more for 2 reasons
1> Rand does not sound as "extreme" as Ron
2> And this is the most important reason. Praising Rand does not require Rush to admit he was wrong.
That was the biggest obstacle we faced with Republicans. Supporting Ron *required* them to admit nation building in Iraq was a failure and a colossally bad idea. Ron wouldn't give his opponents an out on that subject. I don't think he could have if had wanted to. Rand is a little softer on that subject ... But more importantly he isn't Ron. He didn't call out the Republicans on stage in 2007. Nation building in Iraq is in the past and supporting Rand doesn't require Republicans to take the blame for screwing up the country over the past 10 years.
Politically that's probably good. Personally I'd like to rub their nose in it ... but that doesn't win votes.