I think this probably depends a lot on each person's own experiences with the children already in their life. From those children, they may gain an impression of what girls and boys are like!
For example if you already have, or know, a boy who is very active and exhausting then you may hope for a girl! And if you have a boy who is very easy-going then you might hope for another one. Or if you have a girl who gets on really well with other girls then you may wish for another little girl.
Thing is, every boy and girl have a unique personality all of their own. So a second girl might be a completely different child to the first one! And any generalisations you may have in your mind about girls and boys will probably go straight out the window!
So I would not choose the sex. Despite the fact that I have a little girl who I'm perfectly happy with! I would be very happy with any child, whatever the sex.