A female of breeding size and age is apt to produce/lay infertile eggs (slugs) even if she is never introduced to a male. I have had a few females do this. She may not lay them every year and she may lay them 2 or even 3 times a year.
As to which sex makes a better pet, you'll probably get a whole bunch of opinions, but personally, I don't think there is any difference...both are great! You may want to pick a male for the simple reason that alot of breeders are charging just a little more for females and also tend to have more males to choose from, giving you a better chance of getting the morph you want.