Theoretically:

US

Practically:

Neither - both will just run from each other.

depends on their fighter ability, my lv114 160vets boosted +11 km fighters still not good enough......

but if you want to level up fighter, go to US

It's also depended on CV player skills (i.e. dodging, dragging, altitude changing, etc)..

USN has a innate +3 to fighter growth (KM has only 2) and their planes are just plain best at everything.
IE. They have IJN speed and fuel with KM firepower... and armor.

Basically USN has slightely better pilots and faster planes with more fuel.

As a KM cv n00b I would say USN are the best. Fought with many different players but USN CVs make my life miserable the most.

I am prepping to begin the US cv grind ... But having 100 km fighters... Yeah... Us owns them

I have US CV6 with T4s. Easily, the enemy fighters that require my focus are other US T4 Fighters. ;)