Bismark is... the most amazing BB3 in game. SO much fun. But, an extra support slot, MUCH smaller hitbox, and (i think) better turning and speed from the AD is hard to pass up.
Bismark will teach you how to play H39+ (KM BB style). AD will get you to H39 faster... without much skill. So, you'd have to learn KM BBstyle in H39.
Richard, did you ever test the Stalin? I'd love to see a range/spread comparison between stalin and H39.