My little comparison between M80 and DT1350 as my M80 just arrived few days ago.
Instrument Separation
Even tho they are both supa aural, I can still tell the difference between the instrument separation, DT1350 is just a bit better. The overall sound of the M80 just sound a bit too crowded to my ears. The soundstage seems to be bigger on the on the M80 tho.
Lows
There is also a upper low boost on the M80 which I did not really enjoy (if you want to hear bass guitar,M80 is the way to go). Personally I think it makes the sound muddy and a bit boomy. The M80 may sound airy compare to DT1350, I think mainly because it has got that low end boost which enhance the reverb effect.
Depending on what you listen I guess. The bass of the DT1350 may not sound as thumpy but it is tighter and can actually go deeper.
Highs
Strings just sound a bit better on the DT1350, they've got treble boost. I think it is the beyer signature. It can be harsh sometimes.
Comfort & Isolation
Most people complain about the comfort and fit with DT1350, especially the clamping force. I had the similar problem at the start but now, I think the headphone is pretty much fits very well on my headphone. I thought M80 would be more comfortable, but after listening for a period. My DT1350 wins. The rounded pads just feel better. DT1350 also isolate better than the M80.
Overall
M80 is a fun headphone with good packaging, but I just like my DT1350 better. I think M80 will be more forgiving to bad recording. After all, you should try both and compare them yourself.