The difference between dogs is like the difference between people. Is it possible to say who is better, one person or the other? They're just different, even within the same sex.
We live with both girls and boy, but we still can't decide who we like more.
In general, boys are more straightforward, their actions are more understandable, they are good for training, sports. Girls are more cunning, but they are also more malleable and easier to agree with. I think boys trust their owners more and think less on their own. Girls have special days and their whole life is subject to cycles. They can get irritable before these days, fortunately this period is short.
Obedience depends not on the sex of the dog, but on the degree of education and contact with the owner. In this case, if the boy is quick to agree with what is not allowed, the girl will check a few times - and maybe still she can do it.
Sum up, Corgi is not a breed that has significant advantages in one gender or another--the males are just as good with children and other pets as the females, are as easy to housebreak, and are not more aggressive. The experienced breeder will be able to advise you which puppy's temperament meets your needs-they don't want to put the go-go-go puppy with a retired couple who want a couch potato, nor the quiet puppy in the home with 4 children and a position waiting on the soccer team!
With the right relationship, both girl and boy will be equally loyal to their owner.