I don't really have an opinion..... as I've only had mixes. We've gotten both of our pups from a rescue, and they're both German Shepherd mixes. We did look at a German Shepherd that came from a breeder who had financial problems and had to get rid of his dogs, but that just didn't work out for us.
My mom always told me that purebreds tend to have more medical issues. But my last dog, who was a mix, had soooo many issues. He had cancer twice, one as a puppy that he had surgery for, and once when he was 8 that wasn't treatable. He had a very sensitive stomach and allergies. He had so many psychological issues- he had bad separation anxiety, fear of heights, etc. So I don't believe that one is better then the other.