Advice on what dog to get

Question
I have a 6 month old Mastiff/Pitbull mix.  He is really sweet and really good, currently in obedience training.  We want to get another dog, so he has a friend to play with but don't know if we should get a male or female because we are worried about the female in heat since they are inside dogs.  Also, we don't know if we should choose a Lab, Mastiff, Great Dane, Pitbull or any other dog............what would be the best fit.  Can you help me?  
Beau  

Answer
I would definitely get a female and either have her spayed or separate her during the heat cycle.  Two male pitbulls and two mastiffs may become aggressive with each other when they reach around two years old.  As far as the breed to get, it depends on your life-style and how much time you have to spend excercising two dogs.  Out of the choices you gave me, the Great Dane needs the least excercise.  It is a good idea for a dog to have company if they are alone during the day.   A lab needs alot of excercise, but if you get a pup, pick the quietest/calmest in the litter.  Another pit or mix would also complement, ask the litter owner for the most layback pup.
Nancy