I've only had one girl, but I've had her for 9.5 years while the boy dogs we had passed away quite early at 4-5 years. So while I voted I prefer girls, it might just be that I prefer MY girl.
We have a 6 month old boy now who is just gorgeous as well, but the old girl just has a serenity and wisdom about her. I remember breastfeeding my eldest daughter 8 years ago, and Buzz just came over and rested her head next to me and watched.