what dog to pick

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
The simple solution is to get a female and have her spayed.  The combination least likely to cause problems is a neutered male and spayed female.  You would have to be nuts to keep an intact female with no good reason.  I know, I have done it with highly valuable ones that may be selected as breeding stock later. I also have a 12 year spayed Lab that is lively and trim, not fat and lazy.  Here is a quote from somebody on another site that is being beat up for accidentally letting their Beagle get bred.  ''We live in the country and have one neighbor that has a male dog, but he lives a mile down the road, she squeezed through the door as I was leaving for work and took off running, she had never ever tried to run away before, when she wasn't in heat she would run on our land, but never ventured any further, she was probably away for 20 minutes to a half hour, until we realized where she went.''

Two males are more likely to get along if both are neutered.  The biggest factor is status in their litter.  One male and one female always stand out in their drive for dominance.  Put 2 of them together, and you are asking for trouble.  Select a lower ranking puppy from the litter, and it likely will be accepted by another dog.  

As for the breed, I don't think it makes that much difference.  The variation of one member of a breed to another can be more important than which breed.