Welcome to BPNet.
The first bit of advice I'd share with you is to enjoy the snakes at the pet store but consider purchasing direct from a quality breeder either at a reptile show or online. There are some good pet stores out there but a lot sell snakes in poor health, with mites, unsexed, not feeding and so forth. We see a lot of new owners here at BPNet struggling with those snakes and it's awfully tough for a first snake experience. Just something to consider.
My family keeps both ball pythons and boa constrictors (the most commonly seen in pet store type - Colombian boa aka Colombian red tail aka common boa - pet stores call them a lot of names). We love them both. However for a first snake I'd probably go with a ball python. Boas can get a lot bigger so making sure of the gender of your snake is important as the females can get quite a good size. Boas also tend to have a stronger, longer strike range. However, boas are also better able to handle the stress of human handling than most ball pythons (this is a generalization of course).
I think it's wonderful that you are making this decision so carefully. It's a big committment and there are so many types of snakes that it's best to do it this way and get the snake that best fits your family.
Oh and once you settle on a snake, please come here to see about enclosure stuff. The pet store will often sell you a ton of stuff at a ridiculously marked up price. Read threads here and you'll house your snake properly without wasting any of your hard earned cash.