If you're getting these hairless as pets and the possibility as a breeder in the futre, get hairless males. Hairless females lactate very poorly.

If you want JUST pet hairless rats, like m00k said, females are very curious and excitable, males are more likely to just sit there, but pee on everything as they mark their little routes.

Whatever you choose, don't get just get a single rat, they should be with other rats all the time.