20 days ago - Blitzkrieg Wulf - Direct link
General consensus seems to be that the D-12 is better at "extreme" altitudes than the D-9... A lot of people basically just say to pick whichever one has the armament that you prefer.

IIRC, the MG131 has similar enough/exactly the same trajectories as the MG151/20, so you don't have MiG-15 issues, even though the D-9 has a mixed 13/20mm armament.
For the D-12, with the 20/30 mixed armament, you can/will have "Straddling" issues where the 20's will "lead your target" and the 30's will "trail" your target, and nothing will actually hit your intended target.

My vote is D-9 & keep moving up the tree without the 12.