I have both too and I do them in a rotation: oldest to newest and then over again. I also have Mega Step Blast which is NOT on the classics but is good too.
I think the major difference is the complexity. The Classics are more linear and straight-forward. I like that because I can modify my intensity more - I do more jumps, etc. to get my HR really as high as I can. For example - Step Jam (Classics) I have to be careful NOT to go all out in the beginning as I will be DEAD by the end - (dead in a good way). BUT because Power Max (Hits) is HARD to modify (go harder) my HR is the lowest of all of the seven I have... Which is good to know because if I have a day I don't want too much intensity, I can reach for it.
It didn't take me long at all to learn the first 4 (that includes MSB) but FOREVER to learn the last 3 from the Hits DVD. Some of the moves I would get stuck on and instead of going ahead to see how they were used in the combo - I would keep trying to understand them in the "learning part" and they did not make sense but once I did them in the combo - they made complete sense and then I was able to understand what she was doing/talking about. Two BIG examples: up and back in Step Hits and the 360 in Power Max - once I put it all together it was wonderful!
Hope that helps!
Gypsygirl >^..^<