Spinks was more of a durable guy but Foster was a devastasting puncher, Spinks was up there as a LTHVT puncher but behind Bob by a few degrees
I think Foster hit harder but Spinks also had more of a boxing style, and i'm not sure he commited to his punches as much as foster did.
Foster didn't have the luxury of a trainer like "Marty ?" help him bulk up to hvyweight. A Cooney - Foster fight would have been like Cooney vs Norton.