Mine are two sisters but like everyone else says, as long as they get fixed, I don't think the genders really matter. But I think it's better if they're from the same litter... we adopted two kittens from different litters when I was a kid, and I don't remember them ever being as friendly with each other as my cats are.
If I do get one male and one female, do I need to worry about them mating if I get them fixed at 6 months?
My boyfriend's cat got pregnant when she was a little over 5 weeks old. I know boys cats usually reach mating age at about 6 months too. I would talk to your vet about maybe getting them neutered a little early.