I asked V-MODA support about it since a lot of people are wondering and they charge you right away.
The M-80's are amongst the most uncomfortable headphones I have ever touched (the Beats Mixr is probably the worst). The earpads are barely cushioned and the headband is tiny.
That being said, the M-100 is much more comfortable in that regards (well cushioned earpads that are squishy and a large headband). The pad are so squishy in fact that some people's ears (including myself) touch the driver enclosure which may cause discomfort after extended periods of use.
As for sound, from memory, the M-100 is better than the M-80 in every aspect. Slightly wider and much deeper soundstage, better sound isolation, treble that is actually present in tracks, more forward midrange but still slightly recessed, an elevated bass response that is more suited for mobile listening rather than a fairly neutral bass response.
Comfort for the M-80s tend to vary - it took me quite awhile to get them to be actually comfortable, but now they're actually even more comfortable than the M-100s, due to my large ears. I was able to mod the M-100s to have acceptable comfort though.