Can't decide on which boa to get.

[xFenrir]

-I want something docile and something that won't get too big (6 feet max), but will be bigger than my ball.
Colombian BCI (most likely male; females can be 6ft but tend to be bigger). As youngers they can be nippy or hissy (like most baby snakes) but 99% of the time they grow out of it with regular handling, or even as soon as you get them out of their enclosure. Once they're out, they become very inquisitive and don't mind being "out in the open" or held as much as the shyer BP's. My boyfriend and I have nicknamed our boa "the fat slug" because she's big and as docile/lazy as you could possibly get without being dead.
-I want something that likes to be handled more than my ball python.
If you get a BCI, most of the time they'll end up handling you, haha. I've never had a kid, but I would guess watching after a BCI while it's out and about is almost equivalent to trying to keep a toddler out of trouble.
-I like the big heads of the boas. There's just something about it that intrigues me.
BCI's have the "bigger" heads than most of the localities; BCC's are the biggest in terms of overall size, but they have slimmer heads than them.
-I will keep it in a tub of whatever size the boa needs.
You might be better off looking to purchase an enclosure from Boaphile or Animal Plastics. It's gonna be hard (not impossible, but hard) to find a tub that will be big enough for a full-grown adult.
-I am good with keeping the humidity of my ball python, and she has a constant hot spot, but her ambient temps usually stay about 75-85, depending on if the heater is on her. So I need something that can bare a bit of temperature gradient. Or something that doesn't require high temps. That would be even better. If not, I can accommodate to its needs.
BCI's are one of the most forgiving when it comes to husbandry mistakes or fluctuations. Not only that, but they actually tend to like it a tad cooler than BP's; I keep my girl at about ~78F ambient and ~88F hotspot.
-I will probably get a male.

-Not very interested in getting a morph.
Even if you change your mind, hypos/salmons and other morphs are very affordable.
-Relatively low prices would be nice.
You can get a baby BCI for between $40-60 at a reptile expo.