I prefer the D over the Q. The Q seems nose-heavy with that big cannon on it, and when that runs out the twin .50s aren't anything to brag about. The D offers the 20mm with ~30 more rounds than the Q's 37mm and has better ballistics on long-range shots at d400. The D also supplements the .50s with four .30cals and plenty of ammo. The D is slower on climb and level speed because it lacks WEP, but it handles better in the turns and rolls.
And they're both prone to rearward attacks by Typhoons, Mustangs, LA-7s, Nik's, and anything else that can cruise faster than 250 IAS.
The one thing the P39 can do is push 500 IAS in a dive without control lock, which is nice for escaping Spitfires, P-38s, and 109s.