I really liked the AD700 when I first got them and the whole time I had the M50's on the way, I was questioning why I ordered the M50's because I was happy with the AD700's. When it was the only headphone I had and listened to, I really liked it a lot. Once I got the M50's and listened to them for a week or so and went back to the AD700's it felt like something was missing, especially in the lower end. But I liked the AD700's because they had great soundstage and when not comparing to another headphone, the base sounded good, although not boomy head shaking though like others, but it was there, just in a different way. I also tend to like a little brighter headphone and the mids and highs of the AD700's stood out pretty well to me and I liked that. But in the same way (and I think with most audio technicas) the M50s have that also. That made me debate on trying to find a higher end Audio Technica headphone before going to the denon d5000's which are on their way for me to test. But, I couldn't find a good pair of higher end AT's at a decent price, plus I thought I would try something outside of Audio Technica.
To compare them is a little hard because they are very different headphones with different styles and sounds. In my opinion, you cannot really compare one headphone to another because each has it's own sound that takes a little while to get used to (because you have to get used to their sound rather than the sound of the primary headphone you have been listening to). So when I get (or barrow) a new pair I try to give it at least a full day or two at work listening to it before I pass judgement on them. If the AD700 was the only headphone I had, and had not gotten the M50's, I would still be happy with them, but now having the M50's and them being my primary headphone I prefer them over the AD700's. Whenever I go back to the AD700's I miss the M50's some. But again if I were to take my own advice, I would have to listen strictly to the AD700's for a day or two to get their true feel and sound back to where I wouldn't be comparing them to the M50's again.
The M50's are nice because they are more portable than the AD700's, but they are a closed back phone instead of an open back. I ended up getting both within a month or two of each other because they were both affordable and I could compare them that way. A local stereo shop also had the M50's which I tried there and liked quite a bit (this was even before I got the AD700's) but I wasn't ready to spend more than $100 on headphones yet, so I got the AD700's.
I will probably be selling the AD700's just because I don't want to have a mass of headphones and it's more money to go toward a new set of something else. I kind of hate to sell them because I really liked them for the two or so months that they were my primary headphone and being my first real set, I kind of hate to see them go...haha...but oh well...I will be doing the same with my E7 shortly since it has now been replaced with the O2/ODAC combo (although the fiio is much more portable).