I think by the standards of the 1990's, the 20" frame would be the right size for me but now I'm too influenced by Grant Petersen and I don't plan to ride on any technical terrain so the larger size is more appropriate for my 5'11" height.

I've been a dedicated Trek fanboy since 1995 but the Rockhopper frameset really impressed me and I would choose a 22" Rockhopper over a 22" Trek 930.