I own the ath-m30 (little brother of m50) and the ath-ad900x (bigger brother of ad700/700x). I've heard that these headhphones share a very similar sound signature with their brothers so here's my two cents.
Neither the m50 or the ad700 is suitable for portable use. Maybe a case can be made for the m50 but ad700 (i believe they basically look the same as ad900x?) is out of the question. They're huge and open back with a very long cord. I actually do use the m-30 as a set of portable headphones (i braided the stupidly long cord to make this possible) but they seem quite a bit smaller than the m50.
The sound signature between these two headhpones is very different too. Mainly the difference is in soundstage (way in favour of ad700), amount of bass (way in favour of m50), mids (i suspect ad700 is clearer). Basically m-50 is a much more neutral set of cans with a bit of bass while ad700 has a very airy feel and is awesome for classical/acoustic/female vocal. As much as I love my ad900x to death, I do sometimes miss the neutral sound that the m-30 (m-30 lacks the heavier bass of m-50 so I couldn't imagine a more neutral set of cans) provides and go back to them (not very often admittedly). Also, ad700 will be much better for gaming, no question about it, thanks to the soundstage.
If you want good quality portables, you could take a look at the ath-es700. They apparently share the sound signature of ad700 and ad900 and were rated just below the ad900 in terms of sound quality by one person on Head Fi. It's cheaper than the ad700x too I believe. And they look awesome.