Cesar Milan is THE dog training guy - that is 'hot' right now - his methods make sense and are effective. His advice is sought after. He has books, a TV show, and occasionally travels on road shows.
Here is a link to why he thinks mutts are best: http://www.cesarsway.com/dog-care/new-dog/How-to-Pick-a-Healthy-Pup
Answer: "hybrid vigor"
The purebreds we have had, well, we got tired of losing them to their breeds' health issues at ages way too young. Our mutts (ask us about our Mr. Mo) lived the longest, and had the least amount of vet trips out of all of our dogs. But, we love all dogs anyway ;-)