The 1846 is too fine a watch to use as a beater, whereas the MD is much more sturdily engineered. That is not to say that the 1846 is delicate, but I would not want to put mine through too much abuse, as I just feel more protective towards it!
I had the pleasure of wearing a friend's MD for about a month (long story, and it was tough handing it back I can tell you), and I had the impression it was almost indestructible.
So long as you apply caveat emptor, by default you should buy used. Better value, more fun and allows more cash to buy more watches!
Seriously, you need one of each...