I've got a few examples of both. Broadly speaking there are two types of the olive green issue bag- the older type that was slightly heavier guage material, and the newer type which was a touch lighter, and has a 'pleat'- an extra triangle of material sewn into the hood opening so it can open wider and is easier to get into. The DPM version is almost certainly likely to be to the same pattern as the latter, and is thus easier to get into in a hammock than the early olive green bags.
No diifference that i've noticed in build quality, breathability or waterproofness, but if you are buying second hand, then getting a DPM version will mean you are possibly more likely to get a lightly used bag. No hard or fast rule though!