Well it depends on what you want. Both can get to be fairly large. Females over 10ft and males 8ft +.
The only difference is carpets are much more slender and thin bodied. A 8 foot rtb will be alot thicker than a carpet. I love them both. The only reason i lean towards carpets is bc their colors are more appealing to me. But boas are really nice too Ive been toying around with the idea of getting my first "big" snake, but i cant quite gurantee ill have the room for an enclosure of the right size theyll need. The wait is killlllingggg me!!!