My preference has always been air. I don't want to have to flush the system when the time comes to use it. IMO a little water here and there in the system won't hurt anything. I only use antifreeze in the traps.
2015 Greyhawk 29MV
Honda CRV toad with Blue Ox baseplate
Ready Brute Elite towbar with braking system
TST 507 Flowthru TPMS