What Bridgman said.
I just wanted to underscore the notion of the dimensions of the speakers and the configuration of the room. If you are seated along the short axis, you might have issues with the M80's because of their depth. It's not that the M60's are that much smaller or shallower, but each does need some breathing room behind, so have a pretty good idea about positioning before ordering either.
Likewise, the impedance may or may not be an issue. Trying to run the M80's on (for instance) an older, lower-end Sony receiver could cause problems that you would not experience with the M60's.
