I would go with a female as your first boa. If you get the female now and raise her up, and maybe a year or two before she's ready, get your male, you won't have to wait for her to grow up to breed. By the time your male is ready, she'll be ready. If you go the other way and get a male first and raise him up and get a female later, you'll end up having to spend more time raising her up to be bigger. Females may be more expensive at first but you will be able to breed earlier.