You get more experience and credits for destroying ground targets in ground strike missions as a ground attack aircraft or a bomber.
You get more experience and credits for destroying enemy air in a domination mission as a fighter.
Note, the economy is kind of out of whack at the moment and the credit gains are abysmal, when you hit rank 6-7 then your game is going to come to a grinding halt for a little while while gaijin tries out both extremes of the spectrum ( Before you hardly earned experience and given credits like candy )
Joysticks are best used in full realistic battles or historical, the arcade flight models are too, well, arcadey for joysticks.
The British air is fairly dominating, just like the japanese can be because they're island nations, island nations tend to have a rocking airforce and naval branch of the military, the game is intending Air superiority to the british, naval to the japanese, land to the americans, and the others being a mix of branches but not as specialised as these three. So until Warthunder releases land and navy, yes the british will feel a little overpowered.
Crew upgrades are passives that affect how the crew perform, reload speed determines how quickly you can rearm your guns, repair speed lowers the time you sit on a strip waiting for a repair, G-force increases the amount of G's your crew can handle before a black/redout etc.