Personally, especially as pets, I prefer to acquire a baby.
Adults are harder to "tame" if they behave in a way which is not to your liking.... if they are not accustomed to really being handled, that could prove to be a pain.
I will again re-affirm- RioBravo are highly reputable American breeders of pure locality boas... amongst the "Boa people", there's hardly a person who doesn't know them.
That being said, if they will miss out on this opportunity to sell to you because they usually don't deal with individual sales on this magnitude... well... it's kind of their loss.
Cuttingedgeherp are excellent- Vin Russo has published the most comprehensive Boa book so far and I recommend it to anyone who wants to keep Boas.
The adult Hog isle looks great- from here on out it's your call whether you prefer a baby or a sub-adult/adult.
Good luck