Wanting a second staffy but boy or girl?

We have an eight year old bitch (been spayed) with an unbelievable temperament, of which I think we would struggle to get another the same. Never wanders off, gate can be left open and she has a look out but just wanders back into the garden.
My thinking is though, as she is an adult dog would she sort of help with the training of a puppy?
We are undecided on whether to get another girl or a boy so any advice or experiences appreciated. I'm sort of swaying to another girl but my daughter wants a boy.

This was a question that I partially asked a while back in this thread
https://staffy-bull-terrier.niceboard.com/t67399-considering-a-friend

I got another girl, Opium.
I consider myself very lucky that Nifty is so laid back and forgiving because Opium is rather turbulent ...
Boy or girl was a question that I asked Nifty’s breeder and she confirmed that a boy would generally be much easier to integrate.

Opium was a 9 week old puppy when she came home and Nifty did nanny her. I think she set a good example on the house routine so Opium followed naturally.
One thing I did insist on, with the acquisition of Opium, was that Nifty could meet her first to make sure they got along. This might be difficult to negotiate as I can put myself in the position of a breeder and having an adult outsider dog come and meet one of your young puppies - not everyone would be comfortable with that.

I'd say boy, but a lot depends on the temperaments of the new pup & its parents.

To be honest, so many people have two of the same sex & have no problems. Apparently two girls are more likely to be unforgiving if they have a spat, but I've not seen any proof of that.