We had this same question when we were getting another dog. We started out with a male dog Bandit, and we wound up getting a female Nala. At first there were issues and Bandit was very disinterested in her, but now they get along great and when he is tired of playing with her (and her endless puppy energy) he hops up on the couch where she can't get to him.
Most people we talked to suggested two opposite sex dogs.