A book could be written about ENY, but in a nutshell each plane and vehicle are assigned a value based on how "good" they are/how easy they are to fly/how versatile they are.
In a general sense, the more "late war" the plane is, the lower the ENY is because it is 'better'. There are some outliers, such as the Bf 109K-4 because it has 1 30mm cannon and 2 ~.50 cals. The ENY is higher because it is late-war but still a bit tougher to fly and do well with than say a P-51 or La 7. Then you have a plane like the Yak-3 which will do anything you tell it to, but has a very limited ammo count so the ENY is higher than it should be.
As for changes in some planes, they may not be easy to spot differences between models, but often some have different engines that allow for more Horsepower, or a different gun setup (30 cal vs 50 cal) or a different amount of ammo (Spit V is a good example of this with only 120 cannon rounds).
Lastly you have your "perk" planes, which are planes limited by earned points so they are not super abundant in the arena. These are planes like the jets (Me 262 and Ar 234), rocket (Me 163 interceptor), and various super-late war piston planes like the F4U-1C (4x 20mm on a Corsair), F4U-4, Tempest, B-29, etc). Basically if these were free they would be the only things flown in the Melee arena.
I do suggest events. FSO is on Fridays at 11PM EST and is a 1 life event. Scenarios run every few months and are generally 2-3 lives in the span of 3 hours on Saturdays. They are much more immersive than the Melee Arena in that there are historical match-ups, even numbers, and pretty historical missions.
