I have both headphone, the COP's for a couple of months and am just burning in the M-80, so I can't give a final impression on sounds, though they are very different.
A lot is down to what you want the headphones for and the kind of music that you listen to. The COP's are quite big for a fashion headphone and not really practical for travelling around. Because the COP's are close back, with the pleather pads, they get extremely warm when walking about (I changed the pads to velour and they are much better and more comfortable. I also found that I got a pain under the ear, by the jaw line from the COP's which I eventually got rid of by stretching the COP headband out (it was much better after and really nice with the velour).
The M-80's I found to be comfortable out of the box, with some miner adjustments to the headband. You don't get the heat build up of the COP's. Because the M-80's are on ear headphones, you do feel the headphone on the ears but it isn't uncomfortable. The M-80's are much more portable than the COP's (one of my major reasons for buying them).
The COP's are unsurprisingly much better at isolating sound and there is very little sound leakage, the M-80's don't do a bad job but are not in the same league.
Build quality of both headpnes is exceptional and I have no problems recommending either. I really have to commend V-Moda on the build quality of its product, Bayerdynamic does a good job but the M-80 just takes it for me.
You get a lot more extras with the M-80's, which come with a lovely molded hard plastic case, 2 cables and a small carabiner. The COP's come with an allen key and a 1/4 adapter (you don't get an adapter with the M-80). You can change the backs on both headphone and have different colours and designs. Also the cable is detachable on both, so if something goes wrong with the cable it is easily changed, The COP's have a lock, which can make certain other jacks hard to fit in, so it is something to be aware of. Likewise the M-80 has thin jack so not all jack heads can fit.
I have actually messed around and changed over cables from the M-80, and the sound does come over as louder, than the M-80's
Sound wise. I found the COP's quite horrible to begin with and nearly returned them. I found after a couple of hours the sound started to get a little better and after 60+ hours the they are much much nicer.
The big thing with the COP's is the bass. You get a custom sound slider with 4 different settings, which effect the level of bass, quite dramatically (I often have the slider open between settings as well) and found that you can get an even more varied sound. Generally I would say that most people will go between the 2 middle settings, where the base is at its best.
The mids are a little withdrawn, though this isn't a massive negative but I would like them to come out a little more but they do improve with listening. I wouldn't say that the mids suit all kinds of music
The highs are nice and really don't overload the sound, which I like.
Overall I would say that the sound of the COP's is good, they are not the most detailed headphone but I found the sound to be quite smooth and the adjustable bass is nice. One of the things I have also noticed with the COP's is that the soundstage has improved enormously with a good burn long burn in and continues to improve over time.
As for the M-80's sound, I am not near to being able to make a final though on them yet but here are my first impressions.
The bass on the M-80's feel a lot more punchy than the COP's but the COP have more power and are more in line with other fashion headphones. The bass on the M-80's is a little more even when compared to the mids and highs.
I do like the mids, though I find with certain music such as trance the upper mids are a little over the top for my taste. I have found that they do sound great with vocals, listening to someone like Kate Bush is a pleasure. I also thought the headphones sound much better with classical than the COP's due to the mids more even sounding nature.
Again the sound of the highs is decent, again nice, not going to over the top.
I didn't find the sound stage to be that great compared to the COPs but hopefully that will come out, with time. The sound is also a lot louder with the M-80's than the COP's, when played though an MP3 player or phone.
Really the sound is very very different with the 2 headphones. The M-80's are much more neutral than the COP's, so a lot is down the sound you like and the kinds of music you like. I would say that the M-80 is more accessible to different kinds of music than the COP's, which prefers and is better with more bass heavy music.
In the end I would say it is down to what you want from the headphone, as to which you should buy. If you want to travel around town, listening to music, like pop, classical, jazz etc then the M-80 for me are better. If you like much more bass heavy music, then the COP's could be better but they do need a lot of burning in to get the best out of them.