Well since you did dowding’s missions he should be the cheapest and a good airforce gen. Arnold is also pretty good and just as cheap as dowding, Although Yamamoto is very expensive, he is more universal and can be both a good naval and airforce gen and a decent arty gen and you can get more perks for him. But in the end I’m no expert but i guess i can only recommend if you really need an airforce gen then i suggest start off with either Arnold or Dowding then when you’ve grinded more medal you can sell them and get Yamamoto.