Based on frequency characteristics only, my fairly extensive listening experience with these phones puts the M100 above the M80 when you don't have the option to EQ either of them. The high-end rolloff on the M80 makes them sound fairly dull next to the M100.
When you can EQ the phones, it turns out that M80 is a bit easier to EQ, and the difference between them becomes much smaller. In fact, it is possible to EQ the M80's sound to mimic a much more high-end phone, whereas the M100 has a distinct sound signature that cannot easily be changed. Of course, using EQ means you have to do something special with your playback system (use an app on a smartphone, Rockbox, or a laptop).
None of the above talks about imaging, though. I'd be curious to hear other's opinions on a soundstage comparison of these two headphones.
Oh, and in my case, I've got two pairs of the M80s and one pair of the M100s. The M80s hit that sweet spot of sound quality, portability, sound isolation (some but not two much), and looks when I'm out and about.