The OS on the air gapped system is not that important, since you don't have to deal with regular internet threats, and anybody who wants to attack you will use

The OS on the air gapped system is not that important, since you don't have to deal with regular internet threats, and anybody who wants to attack you will use 0-days anyhow. But I think that the air gapped computer should have a different OS than the computer which writes the USB sticks (and is connected to the internet). Just to force the attacker to burn two 0-days.