I simply do not see a lot of females offered for sale as potential K-9 prospects. I have tested a few and one Dutch Shepherd was awesome. The vendor "claimed the dog was two, but based on her teeth and body she appeared to be about 4 and had several litters. Awesome dog, if I wanted a bitch for breeding working dogs she would have been great.
I have owned some awesome females for sport and I enjoy working a female in IPO. Hence, my new GSD pup is a female. When it comes to LE work, we do not spay or neuter dogs. Some females will get a little flaky when they come into season, it will also affect our male dogs. Not a big deal, just one reason. Males are generally larger, have more aggression and do not go into heat.
There is the belief that females are more protective and will not leave the handler to engage someone a distance away, I do not believe this. I think a hard female will work just as a male will. We just do not see them offered for sale. I do test them occasionally and test them the same as the male dogs. The availability of good females is very low if not rare in my area. We do have a super single purpose lab in our narcotics unit and she is great.
For patrol dogs, I like the size, aggression and working abilities of male dogs. I am testing dogs this week, if a super female was available I'd take her. But, as correctly mentioned, females like that are often held back for breeding.
“Beware of false knowledge; it is more dangerous than ignorance”. George Bernard Shaw