Own both headphones and whilst there are similarities when it comes to build quality (which are both very good), in most other ways they are very different.
For my the COP's are not very portable. They play fine on a portable device but the headphones are just too big an clumbersome to carry around all day. I tried it in London and whilst they are comfortable to wear on the head*, if you have to take them off of the ears, they are exceptionally impractical (Around the neck, it feels like wearing a neck brace),
*Whilst the COP's are comfortable, I found that the clamping pressure, pinched under the ears on the jawline and I had to really stretch out the headband to relieve the pressure.
The M-80's are better for wearing around out, due to their size.
When it comes to sound leakage and isolation the COP's are easily better than the M-80. I found the M-80 leaked noise, quite a bit and the isolation isn't good (I can hear pretty much everything when walking around wearing them). The COP's due to their closed back design are much much better and when wearing the headphones there is very little to no sound leakage that I have noticed.
One of the disadvantages of the COP's is the heat build up, due to the pleather pads and closed back design. This was one of the other areas where I found the COP's impractical when walking around. I ended up seating bucket loads, when wearing the COP's out and even when indoors the pads get warm. In the end I replaced the pleather pads with velour pads, which are more comfortable and don't have quite the heat build up. The M-80 on ear design, means that I didn't have that problem. They are not quite as comfortable as the COP's but I have no problem wearing them for long periods.
With the M-80 you get a carry case, and 2 detachable cables, with the COP's you get a detachable cable and an allen key. The M-80 carry case is really nice and practical. The 2 detachable cables with the M-80 have a kevlar cord but can get caught up in knots, something which I haven't found with the COP cable and when brushing up against clothing the M-80 cable can make noise, though I haven't found this to be a major problem. The M-80 cable also has a remote control on each cable, one for iPhone, one for Android. The COP cord is thicker and locks onto the COP headphone (though is quite easy to get off). I generally use the spare cable from the M-80 on my COP's, The sound from the COP cable is a little quieter and it feels like the M-80 cable brings the mids of the COP out a bit more (though this could be my imagination).
The sound of the COP's and M-80 headphones is also very different. The first thing I should mention is the burn in times. The COP's take a age to burn in. I have had them for 3 months and it still feels like they are burning in. To begin with the COP's sound pretty awful, I would give them a few hours to burn in before even trying to listen to them. It took about 12 hours before the sound started to sound decent and a lot lot longer before they started to sound good. I would say it was 50+ hours before the sound stage started to open and there has been a dramatic improvement over the time that I have been listening to the headphones and I new get a real sense of imaging when listening to them.
The M-80's do improve a little with burning in but the sound hasn't changed dramatically. The soundstage has opened up a little but isn't as good as the COP's.
The Bass of the COP's is much more powerful than that of the M-80's but I found the M-80 bass to be a little more defined, having a little more punch to them. Of course the COP's have the slider, which allows for different bass settings. Although technically there are 4 settings, I would say that this, isn't quite true, more often then not, I will have the slider open between settings and I have found the sound at 2.5 to be really good. Generally I would say 1 is too bass light and 4 bass heavy with the sound becoming muddy. Generally most reviews I have seen recommend between 2 and 3 and I cannot disagree with that. As to which bass is better, if you prefer heavy bassy bass then the COP's definitely are better but if you prefer precise bass then maybe the M-80.
The mids and upper mids of the M-80 is much more presence than the COP's, which are quite regressed. Generally I would say that I like mids some where in between the 2 but it is easier to get the M-80 mids down in EQ than to get the COP's up, without getting a little distortion. The COP's have improved with time though and are much closer to what I like, than when first listened to them.
I found the highs of both to be similar and haven't really got a huge preference between either, maybe the M-80's highs are a little bright for me at time but again I generally EQ them down so they are less of an issue).
The sound of the COP's is definitely much warmer than the M-80's. The M-80's in contrast have a much more even sound. I wouldn't say that either headphone has the most detailed sound and these are definitely not headphones for audiofiles but the sound is nice on both.
Generally I would say the COP's are a little more specialised, though with the long burn in I am starting to change my mind a little. When I first heard the COP's I didn't like them with rock due to the mids and upper mids being really regressed but as the sound has come out they have improved and listening to Pink Floyd has become a joy wearing them. Also listening to something with big bass is really good, I listen to a lot of trance and the COP's really suit that kind of music.
For all round listening the M-80's are better because of the mids. I found that listening to the female vocals, classical or accustic guitar comes out at me more.
In my eyes, neither headphone is perfect but both are good. I think the COP's are more frustrating in some ways. The burn in means that you are taking a much bigger leap of faith if you want to buy them and the mids for me, mean that they are an acquired taste. Combined with the fact that they are not that great for travelling around, I wouldn't say that the COP's the M-80 are the better more practical headphone for general use. The big problem for me when using the M-80 though is the sound isolation and leakage, which can be a pain when travelling around a big city.
For all its negatives I do have a soft spot for the COP's, after changing the cable and pads. With the right kind of music, they do sound very good and are fun to listen to.