I've had ball pythons as well as hog island boas. Hog island boas are about my max size, as she weighs more than my cat at the moment.
As far as space, it's easier to find a proper cage for a ball python than a hog island boa if price is an issue and you're not good with diy projects.
Both are great pets. Here's a picture of Valentine who lived with me for quite a while till someone else REAAALLLLYY wanted him.
As a whole, I'd say ball pythons are more docile in nature. Hog island boas can be kind of iffy in temperament some days. Usually they're not that bad but times when they're in shed or maybe a lil hungry they tend to strike out at you. Seeing how far their strike can go, I'd invest in a nice snake hook if you go that route. Not to mention if you ever have to medically treat your snake, as I've been given snakes that had an URI or pneumonia before. It's not the funnest thing to do, and a lil more difficult with larger boas or snakes in general. Just something to think about.
I tend to prefer boas to pythons, but that's a personal preference. If you're comfortable around snakes I'd say go for whichever one you want. I'm sure whatever other snake ends up in your home will be happy it's there.
I've met plenty of people that get boas and suddenly don't want it the first time it strikes or bites them. Completely irritates me, they think they can get them when they're cute and small and when they get too big it's ok to give them away or release them outside. I remember specifically telling a police officer over the phone one time that she can not release a boa constrictor into the woods in the U.S. for legal reasons.
Sorry I went off on a rant, but knowing these things happen all the time is really frustrating.