which one is better? use macbook air on battery power or on ac power while 100%.

Personally, I would say it doesn't matter as once the battery reaches its max charge, the charging process turns off.

FWIW, in Battery preferences set it to Optimize charging and forget it it and don't worry about micromanaging battery use.

Do whatever is convenient for you. Leaving the computer plugged in will not damage the computer or the battery. The MacOS manages the power/charging system very well. Let it do its job. You do not have to micro manage the system.