Let's be claer on the HK - it will easily drive your setup with either M60s or M80s. Not an issue.
The M60s are excellent speakers and the M80s simply give you more at the top end. The M60s wil do fine unless you have an extremely large room and you can apply the $$ saved to a handsome SVS sub. That takes care of the logic part.
if you simply WANT the M80s, no reason to not get them unless your room is small (12x14 or less). The upgrade itch and the "what if" itch can never be fully scratched!!! If either will be a problem for you, now is the time to do something to avoid it. Next we must consider how important bragging rights are to you.
You will, however, find that the M60s are all you really need.
