First off, you can get the hd650 from http://www.razordogaudio.com/ with coupon for about $400 shipped brand new in the box authorized reseller. Mention that you're a headier and you might get a little better deal.
The hd650 and ma900 are quite different. I used to think that the hd650 was about as comfortable a headphone as you could buy. It's not. The ma900 is much more comfortable. It's kind of like wearing a baseball cap. After a few minutes you just don't know it's even there. The sound stage on the ma900 just shames that of the hd650, plain and simple. The ma900 is like having a small pair of speakers in the form of a headphone.
Is the hd650 worth 2x the price of the ma900? Maybe. The build quality of the hd650 is markedly better in all respects. To call the ma900 flimsey is an overstatement. But is seems that way when compared to the hd650. You'll get a richer layering of sound with the hd650 and the ability to scale up with nicer gear.
Compared to the MD the ma900 still holds its own. There are some similarities in tone and overall signature. But you couldn't have a more closed or more open comparison. The MD will isolate everything while the ma900 isolates nothing. The MD has plenty of heft while the ma900 has none. You'll need a good amp with the MD and you'll not need much of anything with the ma900 except a good source. Both are a bit warm and a touch dark. Both great headphones in their own right, just very different.