FWIW, the ventair's have a really nice hat band that's not too elastic and not too tight. It makes the size of the hat a little less critical. FWIW, I'd err a little small for a ventair than a little big. For any other hat, I'd probably err the other way, but these Ventairs really have alot of give to them. OTOH, if you're confident of your hat size, I'd go with the size you know you wear.
Having said this, you want to be as inconspicuous as possible when stealthing, therefore I recommend this one...