Not sure who all keeps Boas and Balls, but I have both and while I've been hissed at by a few BP's will little issue, a boa is totally different and much scarier (at least for me). My favorite pair are Central American's, which are smaller, but are typically more aggressive.
BP's just puff and push the air out of their noses or mouth/nose, but it's a flattening of the body. A Boa will rear up, open its mouth, and push the air out in the classic snake hiss. Not only is it crazy to see, but it's super loud as well. If my guy hisses at me (he does when I go in to get him) and I put him back while he's still hissing, I can hear it clear in the other room. I'm sure there are some good youtubes out there for the hissing boa.