Tough to compare them, imo, but I'll try.
The z7 is much more comfy and has more bass quantity along with much wider soundstage.
Aside from obviously being more portable and wireless, the m2's bass (in wired passive mode) is a bit cleaner but can sometimes sound over-boosted. I think Sennheiser did that to reduce chances of having too little bass (may be harder to use any kind of bass boost in wireless mode depending on the device and software in use).
Both the z7 and m2 have good separation and can bring out the minute details in many songs. So far, I'm amazed how much soundstage and clarity the m2 often has. The m2 is also easier to drive although both can be driven pretty well by my Note 3.
I do think the z7 responds a bit better to amping and are less fatiguing all-around but your ears can get warm and/or sweat more in the z7 due to the much larger, deeper, and more plush earcup padding. I could listen to the z7 almost nonstop but find I have to reposition the m2 every hour or so. However, even if physical comfort were equal among them, the z7's unique sound sig is also less fatiguing than that of the m2.
I'd say the m2 was built/designed exactly like a travel headphone that will be taken on/off frequently or moved and repositioned often; as in motion, on-the-go. They're still comfortable and sound amazing but they can't touch the z7 for long-listening.
