Own the D as well as M externals. The stock on the D is non-adjustable and may be too short; while the MD is just an enlarged version of the M with a different nozzle/stock. Choose the D if you plan to mount a full size marksman scope - the MD for a magnifier or LPVO. Of course, you may choose to replace the plastic externals anyway. Just be aware both are heavier than they appear.
My advice would be to buy the M variant and use the money saved to purchase the Semi lower, that way it can switch between normal and DMR usage with a nozzle switch or Sabre NPAS. You can always install an extended inner barrel under a mock suppressor. Good luck.