True, you will get more meat faster...but mine will taste better and have texture... LOL. Of course, I'm not raising them just for meat either. That's just a side benefit. And Cornish X aren't a dual purpose breed.
American Dominiques are great heritage dual purpose chickens. Our cockerels dress out at 4-5 lbs each at 24-26 weeks, and are delicious. The pullets and hens are smaller, but still lay a medium - large egg, and are economic to feed.