The Pro700 look almost identical to the M50: in size, shape, style, and features (folding, 180 turning, etc). Except for the color (they're silver). The construction seems a bit less solid, however, but that might not be accurate. They are both the same plastic.
Here are the major differences:
The padding on the Pro700 headrest is not as thick. The padding on the ear pads is a full 1/2 as thick, which is really noticeable - the M50 is way more comfortable in that regard, to me, and the isolation is much better. Others I notice find the M50 uncomfortable, and they may like the Pro700 because it is less padded and not as tight on the head. It may also sweat a lot less, because there is a lot less padding there squished against your face.
The ear cups of the Pro700 are circular, rather than oval. This may give them better soundstage, though I didn't notice that. However, the oval ones fit my ears better, the circular don't fit my ears as well. Again, personal preference I guess.
Now about the sound difference, which is vast:
The sound is much darker on the Pro700 than the M50. The bass is less, but more controlled and less bloated. That's a positive, in my book. But the bass is far deeper and more powerful on the M50. This is a major difference between the two, night and day.
The highs are much better on the M50. And I don't consider the highs that "great" on the M50, so I really don't like the highs on the Pro700, personally.
All in all, it was an instant dead ringer that the M50 is a better headphone, IMHO. Didn't take me but 30 seconds to get that. But they are vastly different from each other, besides the overall shape. If you like the differences I noted, then I would say the Pro700 is an excellent pick for a cheap but quality headphone. At $125 new or $75 - 90 or so used, I would say it's great - if you like what I mentioned, I don't.
I ran them with a Total Airhead on High Gain and an iPod Nano for testing purposes.