If you want a female, go with the female option.
If I'm not mistaken, Jeff makes cage extensions and if she grew larger than expected you could pop on the extension.
Boas are fairly consistent in temperament, generally and things can change with age no matter how they start out. A mellow snake at the start can get a bit sketchy later. BCI boas seem to be less touchy than the BCC types, but again everything varies from one individual to the next.
My retic being a wonderful example of starting out placid and turning into a jackass at 3+ years old.
That male looks awesome though.
Go with your instinct!