The M80 bass is fairly neutral, but strong and solid. The M50 bass isn't as tight, but gets slightly stronger below 60 hz or so, and it's quite endearing. The M80's mids are very neutral, which the M50 is not. Much prefer the M80 there. The M80's treble is somewhat recessed, but with a treble boost comes up to hi-fi standard with no irritations, harshness, sibilants - i.e. near ideal. The M50's treble is recessed somewhat in the lower part and has some sibilants in the area just below 10 khz. Although the EQ'd M80 treble is far better I think than the M50's treble, if I had to live without EQ I would take the M50. But where you hear people say that EQ is bad and/or causes distortion and therefore the un-EQ'd M50 is a better choice than the EQ'd M80 - that's not true at all. Ya gotta listen and decide for yourself. And you will read many comments about comfort issues with the M80, so if you haven't seen it, watch Tyll in his demo of the M80 and how to bend the headband. It fixed my comfort issue perfectly.
Edit: I cannot EQ the M50 with my i-devices because the mids and treble are uneven, and it would help one thing and make another thing worse. The M80 is one of those rare items where a simple EQ actually works. But try without it, and if you get a sense of dissatisfaction with the sound over time, try with it before you give up. Most people just give up.