My ATH-M40X's are very, very comfortable.
A bunch of generalisations:
The M30's are voiced more for vocals, and the earcups seem to be harder.
The M40's are very, very neutral. Nearly perfect. The earcups are very soft and very comfortable.
I haven't tried the M50's, but they're supposedly bass-heavy so not so good for recording. Apparently the ear cups are much the same.
For me, the M40's are worth the little bit extra just because of the swivelling ear cups and the detachable lead.
Honestly, I don't feel that any other headphone gets close in this price range. Maybe try some foam under the cups? Or order the M50 cups?