o.o There is no reason for a boa to eat rabbits unless you WANT to. They do just fine on jumbo rats.
Ditto. My roommate recently took in 2 adult BCIs: Approx 5.5 ft and 7 ft each. They both came in a bit on the skinny side due to their previous owner's feeding regime. We have them on a regime of ONE large (not jumbo) rat every 2 weeks, and even on this more conservative schedule, they have both gained weight and girth just in the couple months she's had them.
I've also heard it said by a breeder over @ iHerp that she doesn't use anything larger than a large rat for her boas, even the large adult females, and they maintain perfectly.
Remember, boas are easy to overfeed. Most boas in captivity are, indeed, overweight. They don't need as much food as people seem to believe. (Plus, don't forget about dwarf boas, who stay even smaller and, likewise, need even smaller meals.)
Sorry, got a little OT, lol. My vote is on the Carpet. Yes, they are nippy as babies, but most grow out of it pretty well. I've met some really cool and calm sub-adult carpets. They are really fun to interact with.
One thing to consider... Do you want a couch potato snake, or an all-over-the-place snake? Carpets are VERY active. Boas can be pretty active, too, but they're a lot more laid back and more likely to sit with you during handling time than a carpet.