I own both and have used both for office use.
The Mad Dogs absolutely need a good amp to sound their best. I had used both an ALO continental and a Fostex HP-P1. they needed the power. The MD at their best is better than the Momentum's best. They have a larger soundstage, better imaging and a bit more balanced FR (there's still very strong bass but a bit less low mids/ upper bass presence compared to the Senns. bit more neutral but still dark overall compared to, say, HD 25).
But the Momentums sound similar either amped or straight out of an ipod/iphone, so the convenience factor to drive them straight from a DAP and skip the amp is quite high. and with the built in volume/playback control in the cable, they offer another level of convenience not reached by the MD.
If you want the best possible sound and can afford a powerful amp to get there, get the Mad Dogs (you may just like them enough to keep them as your main headphones). But if convenience (fewer parts in the rig, inline controls) is important, then get the Momentums. The difference in sound is mainly one of soundstage size and a bit better balanced bass/mids response. I've kept the MD for home use but use the Momentums at work.