Many variables to chose one over the other, but it all boils down to personal preference. For us, Its about family. For our price range, having the bed over the cab for our daughter is nice. There are some Class A's that have bunk areas, but they are usually right next to the master bedroom. We prefer to have the seperatio so that we can have our "alone" time.And the Class A's with bunks are generally higher price than the Class C we have.
Other thoughts are that even though I have not driven a Class A, I have heard from those that have, that driving the Class C was more comfortable to them. Also, Class C's usually sit lower to the ground than Class A's. Which is better for those that might have trouble climbing stairs.