I have the HD700 and have heard the MA900, I got a good impression of the MA900's character testing with games/movies/music (tbh I've used the HD700 longer so far but only for gaming, so far!).
The MA900 does have more air to the soundstage (10/10 positional imaging) and has a bit more of an "out of your head" feeling, the Mids were very nice IMO and the sound was not fatiguing which was also combined with lightweight comfort. Pads make contact with your head and touch your ears, but the clamp is light and the headphones so light that you might forget you're wearing them. Bass starts warm, but rolls off extension into subbass and lacks treble sparkle and extension... this has the pros of being pleasant and not fatiguing, but it lacked punch and excitement while streaming Dredd on Netflix and generally for my tastes the MA900 was a bit tame. YMMV, mad thought the bass was great.
The HD700 has a quite full-bodied sound, bass has nice extension and presence but not "OMG" taking over/away from the whole sound. Also has nice mids, treble has an unusual mix of overall not being very bright except for some sounds like cymbals have sparkle, it is exciting but some people find the uneven sparkle spots weird, I don't mind it and feel it was somewhat intentional even if a bit colored. The soundstage and positional imaging capabilities are very good, depth is perceivable, however it has that full-bodied sound instead of that airy-character and imaging is maybe 9/10 for distinct position, some people might find it more "real" and less exaggerated. Weight of the headphone is just shy of double the MA900, but it's still very comfortable and distributes the weight very well so you don't feel a particular sense of pressure, the sculpted earpads fit free of my ears perfectly and the surface texture is one of the least irritating to my skin. Aesthetically gorgeous. Oh, and IMO it would be foolish not to have an amp to bring out the life of the HD700, also the plug is the bigger 1/4" size only and won't fit in portable amps. I use the creative X7 as my DSP/DAC/amp and that combo works admirably (and I play with the EQ), you could also use a home theater receiver, and the HD700 thread has amp suggestions. The HD700 is a big boy headphone, don't let the $100 sale price difference you found confuse you about the quality of the system needed to bring out the potential of the HD700, but when you do the HD700 is on a different level from the MA900.
The HD700's full-bodied sound might be more to your taste, neither have strong bass emphasis but the bass has plenty of quality to appreciate, and the MA900 might be better for hours and hours of gaming. Have you heard the Q701? Those are air-y too, but not bass-shy (just not emphasizing the bass). Either of These don't really have more bass than the Q701, especially once the Q701 pads start to break in from wearing and the oils from your skin, the pair of Q701 I passed along to ZROleaf had great bass presence and we were both impressed with the thunder sound in certain parts of Destiny gameplay (my Q701 were over 2 years old). The MA900 may have a bigger upper-midbass hump emphasis than the stock Q701, but the Q701 and especially K712 had deeper bass extension and had a more exciting sound (sparkly treble helped that too). The HD700 has good extension too, the bass bump isn't really higher but has that thicker body I mentioned.