The designer cross breeds are potentially very unpredictable with regards to size, coat type, behaviour and health etc.
Lots of people have them and love them. But lots end up bigger than people expected, or more hyperactive etc etc. And the price tags are usually a bit scary as well!
There is more predicability in a purebred. Which is especially good when you have kids. Good surprises are lovely, but bad ones are harder to live with.
And I'd expect that differences between girls and boys are not as significant as differences between individuals. If a good breeder is happy to pick a pup they think suits you and your family they are probably pretty close to the mark.