I have a couple of full size griptilians, one is D2 and one is M2, both plain edge. I am highly biased towards a plain edge. I've found that the D2 and M2 blades are pretty close in actual use and I really like both. If I were making the decision you are, I would go with the D2 plain edge and never look back. Both are truly great blade steels, you must decide if you prefer serrations or not.
Bob