What about the BK-37 vs. the NS-37?
In the context of the original question, usage as an air to air weapon the NS-37 hands down - higher ROF, more ammo per gun and (in the Yak-t) using HE rounds instead of AP.
But that's not a fair comparison, as the BK-3,7 is a AT gun fitted to a dedicated tank buster. Comparing the NS 37/Il-2 combination to the BK-3,7/Ju-87G is that one sided anymore. But in AH, the NS-37 still wins:
While having a somewhat slower muzzle velocity, the trajectory of the NS-37 is still flat enough. The BK has a substantially higher armor penetration under ideal conditions, but the nature of the
Hartkern ammunition makes it very sensitive to even small deviations from the ideal angle of impact (90°). And the NS-37 can still dispatch about any tank in the game. Finally, the higher ROF and much higher ammo count of the NS-37 in the IL-2 makes attacking not only tanks, but also small fast moving ground vehicles like the M3 much, much more easy.
Similar to the 40mm of the Hurri D (and most probably even more so), the BK-3,7 of the Ju-87G is much more a specialist's weapon.