If you are going to bring a new animal friend into your home then please, please, please, adopt them from a shelter. If you want purebred's you will find many at shelters. Also, there are many rescue groups dedicated to certain breeds.
To be more direct about answering the question. I just don't think it matters. The animal is also an individual. You can get ten Pitbulls in a room and they will not all be the same. In fact, you can get ten Pitbull siblings all in one litter and they will all be very different from each other. Even when it comes to health issues there are no guarantees that one is better or easier to handle than another.
There are many variables to consider with a question like this. What is the intent?
If you can see that there is someone in that body (like a soul) then the issue of purebred or mixed is secondary to the bonding process of accepting an animal into your life.