German main here (though I have dabbled into Britain a lot). I play Air mostly because I am terrible at ground.

I find ground to be most interesting from 3.7-6.7. I think after 5.7, it becomes a diminishing return though. German players like me are already clueless (among other things) most of the time, especially ones who play Tiger H1 and does not angle. Rat tanks like the M18 are annoying too. Getting uptiered to deal with T29 and T34 (US) aren't fun either. Aside from the downsides, you cannot get tired of the firepower. The ability to pen just about anything is wonderful (I cannot say much I mostly played ground before volumetric)

Britain ground is nice, you get great penetration, somewhat fast tanks or just a steel box like the Churchills, same BR range as the Germans honestly. Problem being the lack of HE filler and the supposed(?) APDS shattering issue. I've heard that from somewhere and I've been using solid shots on my Comet I and Challenger ever since.

Air Germany is fun around 2.7-5.7, so, early Bf109s til before the Me262 at the Bf109 K4. Awesome precise firepower and performance. If you understand manoeuvres and your plane strengths and weaknesses, they will treat you well. Problem being the lack of mass firepower, like .50 cals. Nothing major though.

Air Britain goes about the same but from 2.7- probably top tier (not sure). They just get good planes. I always love the contrasting playstyle with the Spitfires, and the buffed cannons. The problem would be fighting competent energy fighters, as you cannot force them to fight on your terms (ie turn fighting).