It's pretty much a toss-up for me, I have a number of both. I started with Bci and they're still probably my favourites, if for no other reason than I've kept them longer and my first snake was my male Bci. On the boa side, I have 3.5 Colombian redtails (Bci), 1.1 Dumerils boas, 1.0 Colombian rainbow boa and 1.2 Solomon Island ground boas. On the python side, I have 0.1 coastal carpet, 1.0 coastal X jungle cross carpet, 0.1 diamond X jungle cross carpet and 1.0 jungle carpet, plus 1.0 ball python and 0.2 burmese pythons (one albino, one normal phase). Just to round things out, I also have 1.1 normal phase corn snakes and 1.1 snow corns. All of them have their own special appeal, I like the more intricate patterns on my boas and really like the python faces and bold colors - especially the carpet pythons. One of my burms is an adoption (the normal phase) but she's very sweet tempered and my albino is a beauty, at least, she is in my eyes. *grin*
The Bci make great pets as they are typically docile and their requirements are pretty straight forward. However, the same can be said for the ball python (a popular first boid) or carpet pythons. I've found my ball python to be sweet tempered (never so much as hissed at me in the ten or so years I've had him) and he feeds well. Captive bred ball pythons make better pets IMO and tend to have fewer eating problems in my experience. The carpet pythons are extremely variable and all of them are just gorgeous. Out of the four carpets I own, only the jungle carpet python has been skittish...but I've only had him 5 days, so he's hardly settled in. Dumerils are also great boas and no more difficult to care for than my redtails. The Solomon Island ground boas are gaining in popularity (ONLY get captive bred if you go that route, wild caught SIGB can be temperamental and picky feeders). That doesn't even begin to consider the rosy boas or any of the sand boas (all smaller, all beautiful and all pretty docile). The Children's pythons and Spotted pythons are also great from what I've read, though I don't own any......yet. 
It's really a matter of what appeals to you and what you can care for. If you don't want a snake to exceed say 4-5 feet, a boa isn't for you but a male ball python might be...or a rosy boa, Children's or spotted python, or Solomon Island ground boa (or one of the other Candoia). If size isn't really a factor, then you might not have a problem housing an adult Bci with a male staying around 6-7 feet, females a bit more. There's even a good deal of size variation between the carpet pythons, so you'd want to research the particular type before purchasing it. For obvious reasons I don't recommend Burmese pythons as a pet except for those who are willing to dedicate a lot of room, time and expense towards caring for a giant snake. Basically, there's a lot more out there than just redtails and ball pythons. 