brazillian rainbow boa or common boa?

Thats a rough call unless you narrow down your common boa. Mexican Boas (B. constrictor constrictor) cann bite, as will Black Boas (B. c. longicauda) . Red Tail (Boa constrictor imperator) and Hog Island Boas are less likely to bite. Colombian Red Tails are more likely to bite than Red Tails from Brazil or Surinam. I wouls say a Brazilian or Surinam red tail boa and a Rainbow make about equally good pets. Hog Island Boas, extinct in the wild but still captive bred, are somewhat smaller and virtually never bite. Backwater Reptiles has them.

Rainbow boa's are not good beginner snakes even with lots of research. They are generally more aggressive than regular boas, not always, but generally. Regular boas can be aggressive, but mine have calmed down much faster than my Brazillian Rainbow Boa who is still pretty snippy going over on a year old. Rainbow Boa's are also probably going to cost you more for the snake, I paid about double for my BRB than my regular boas. The humidity generally needs to be hirer for BRB's than regular boas so depending on where you live that could be a considerable challenge. I love my BRB, but I am glad I had a few boas before getting her.