The main difference between the M80 and M60 has been said by Alan from Axiom to be that the M80s can play slightly louder with low distortion and have slightly more extension in the bass, while the overall sound was judged to be nearly identical in blind listening tests. Some board members have compared them recently and reported that they heard more significant differences.
When I was deciding on 60 vs 80 Ian had told me that the sonic signature is very much the same but the 80 will go slightly lower and be able to handle much more power and play louder.
You can't really go wrong with eiter one but unless you have a large room or like playing over 100db consistently then 60's should fit the bill. Of course, like JohnK said owners have recently posted different opinions about the 80's actually giving you a little more detail and improved sound at lower volumes.
This past weekend I put in a Phil Collins DVD and at the begining he does a drum solo, then another drumer joins and finally a bongo player. Let me say, this 10 or so minute drum fest was really enjoyable and I brought my system to a peak of 113db very very cleanly and detailed.
The choice is difficult but remeber if you buy 60's you may always wonder.... What if I got those 80's, don't underestimate the upgade bug. I've had it myself but truly love my 60's and the 60's are visually more appealing to my tastes. Good luck!
