Very hard decision!! They are both amazing pets. I think it really comes down to what you are looking for in terms of size, maintenance, and space. Balls are relatively small, do great in smaller enclosures (I'd keep average males in a 30gal, although big females really should have a 40gal), and don't require much upkeep. Boas have potential to exceed 12 feet (although 10 is about average), may require a 6ft long enclosure, and crap like cows. BPs are a pain in the neck to feed, and while boas are usually semi-aggressive feeders, they eat a lot, and food definitely adds up.
Very different snakes in terms of personality and husbandry, but they are both quite docile, although you always have to be aware that they can bite, and while it's highly unlikely, it happens. As a general rule, I say that if you keep snakes for more than a year, you're bound to get a tag or two, even with more docile species. With BPs, a tag wouldn't hurt. With a big boa, however, a tag most definitely would hurt.
I'd say, if you'd like a larger snake and you have the time, space, and money for a boa, and you are up for handling a very large snake, go for it! But you can't go wrong with a BP either. I just got a new little ball girl, and I swear, even after two years in the snake hobby, I still get a rush like she's my first.