Personally? I prefer boas. I enjoy their personalities more than that of any other snakes I've worked with. Not to mention they are among the most beautiful snakes out there, they grow to be a big but not unmanageable size, and they have excellent feeding responses.
However, boas are not for everyone. Whereas ball pythons are entirely harmless, you have to keep in mind that big boas are potentially dangerous and make accommodations to ensure that you are safe. Adults boas are powerful animals. This can also make handling daunting. If you are looking for your first snake (?) and you aren't sure about whether or not you'll be comfortable with a big snake, I'd recommend a ball python.
Boas are also more work than ball pythons. They need very large enclosures (which can get to be really expensive and take longer to clean), they need large food (also expensive), and being that they are much more active than ball pythons I find that boas require more handling than ball pythons and can be somewhat of a time commitment. Of course handling a boa is the best part, but it is something to consider if you aren't home much.
But then on the other hand, ball pythons are less inquisitive, more nervous, and they can be really difficult when it comes to feeding them. Boas and balls are both great snakes, but they are very different.
Overall, I would recommend finding a place where you could handle an adult ball python and an adult boa. They are VERY different animals with very different personalities, and the best way to judge which you'd like is to spend some time with both species and make a decision from there.