Having cared for a lot of both, I would say that Rainbow Boas (brazilians and colombians) generally have a lot higher tendency toward aggression than Boa constrictors... I've dealt with babies and adults of both species and with the same amount of handling etc the rainbows are usually more nippy.
Not saying that all rainbow boas are evil, as I know that with proper handling and care they can be great, i just think its more in their nature to be little demons than it is for red tails
,... so basically, if you get a rainbow boa be sure to handle it enough!