I've had both female and male dogs in the past and I'm not sure that there are any big differences given that my males dogs have always been neutered. I've always read that female dogs are easier to train because they tend to be less distracted. I searched hard for any information about the percentage of guide dogs that are female, thinking that this would be a good indicator. But no luck. I searched for the number of shelter dogs that are male, to see if a certain sex is "less" desirable. But no luck. Probably safe to say, that there isn't much of an advantage for one over the other. I'd say it comes down to three things:
1. Females don't lift their leg to pee ALL over everything.
2. Females tend to be smaller
3. Females tend to be more independant