Should I buy a boy ferret or a girl ferret? Which of the two is generally friendlier?
Personality is individual in ferrets. Spend some time with the ones being offered and chose the one that seem friendliest.
I've had both. The female tend to be more aloof and need to be spayed. On the other hand, the males can be nicer, in general but also need to be neutered. Spaying and neutering reduces the musk smell.
The urine smell is not tolerable from either sex.
Love my Newf
Neither. I doubt you've read Ferrets for Dummies cover to cover https://amzn.to/2DlpSBg and made sure there's a ferret knowledgeable vet near you. You also don't know anything about Adrenal Disease, Lymphoma, Insulinoma or Cardiomyopathy - all diseases ferrets get one or more of in as young as a few years.