Well, the BOBOVR S3 Pro headstrap turned up......
You can see it next to the KKCOBVR ......
Overall......
I suppose it depends on head shape and size, as well as personal preference. But, for me, the S3 Pro is the most comfortable and well fitting. My wife, smaller head, finds it similar. It grips well without feeling tight, that could also be due to having their very good face plate / pad fitted as well.
The ventilation, from the face pad vents, and also the fan within the headstrap blowing down over your forehead seems effective without being overly loud. How good that works out will probably be known better when it is warmer.
Build quality is "ok". It was when I held it in comparison to the KKCOBVR that I then thought, "Oh that reasonable then". What I mean is, the plastic on either is not incredibly think, but probably fit for purpose, meaning the headsets weight needs to be factored when you are wearing it. They do seem comparable in weight.
Easy enough typically, to fit. The 10,000 Mah battery should be decent enough, depending how much use is made of the fan.
Overall, very nice and comfortable, and couples well with their facepad.