I have one dog and thinking of getting him a companion,girl or boy?

My dog is five and we've been thinking of getting him a companion.We've always wanted a girl and decided that it should be a girl.However last week we took him to see a 3 year old girl and she hated him,snarling and growling every time he came to near her.We're not sure weather to get a puppy or a full grown dog and wheather it should be a boy or girl.Please help me as i really do want the best for my little dog.