Slightly less lateral stability near the stall. A very minor effect. On MIL power the M and D-25 will behave very similarly, but the WEP on the M adds several times the number of horses that the WEP on the D-25 adds.
The Major difference between the two is that the D-25 is a real historically significant Jug while the M is a small number production run to a specific fighter group that already had Jugs with similar level performance since early 1944. So in a way, you can pretend that our M is an over-boosted D-25 of the 56th FG.
In the game go ahead and fly the D-25. The M is for players who need the crutch - once you are done with that you move to the D-25 and after that you graduate to the D11.
I fly the M for one reason, and one reason only. It makes Pee-51 drivers dive to the deck when I am co-alt or above. I scream like a school girl when they run (elbows in) all the way home...
I find the D-11 to be a good bird, just a little bit unstable where as the others are more solid... YMMV
