The bigger question isn't so much the total room size but how far away you plan on being from the speakers. I will guess not the full 20' nor 16'. So M60s or M80s would work equally as well.
The next question is how loud do you see yourself playing them? In my experience with the 2 speakers in question the M60s at louder volumes sound very similar to the M80s, it is at lower volumes that the M80s have the advantage, lots of detail and still good bass.
If you are at all inclined to any upgradeitis, then the M80s in a custom finish is the way to go. This way there is no upgrade possible, except for thousands of dollars more.
