I started mixing brown sugar and turbinado in my rub together and have been more than pleased with the results. I smoke my ribs at temps ranging from 240-250 degrees for 4 1/2 to 4 3/4 hours.
I was getting frustrated because parts of my brown sugar seemed to be burning yet my ribs weren't as tender as I'd liked them. So after reading on this board that turbinado sugar had a higher burn point I started mixing in a cup of it in my regular rub recipe and haven't had the problem since.
It seems the turbinado sugar "protects" the brown sugar from burning. The turbinado sugar is larger in "grain size". I like the crust I get from the combination as well.