Infantry Doctrine is the most obvious choice in a Wehr vs Amis matchup. Off-map arty at 3 cps to break down a wehr defensive position or on the pak if you're getting an M8. Maybe the cost of the off-map howitzer should be increased to 175 or 200 muni's? All units / abilities in this doctrine are good, making it a safe choice and thus the most popular.
Airborne doctrine is the 'weakest' really. Paratroopers and the Para AT drop are very expenisve to reinforce and the RR's phasing through Puma's is still very annoying (Is it possible to do something about that? New Puma model perhaps?). Strafing and Bombing Run are too unreliable for their cost as they're easy to dodge by a skilled Wehr player. Thus making it an almost useless doctrine in the late game.
Armor is fine, just need some slight buffs on the raid ability. 1CP less for that would offer more strategical options for A: getting a jeep and B: Early M8. Pershing, obviously, needs a buff for it's cost.
Strafe is actually one of the best parts about the Airborne tree, and I definatly don't see it as unreliable, the bombing run however is very slow, although it can destroy infantry it can easily be dodged,
I feel like airborne could have a tiny little anti infantry boost, maybe rather than all having m1 carbines, a couple could have the m1 garand and have the standard rifleman damage.
Pershing actually does have good anti infantry, and can witstand quite allot of punishment.