I was in the same boat as you late last summer. I was looking for a 3/4 ton with diesel. I was also a long time Chevy truck guy. This is what I found:
GMC Denali - didn't like the Duramax. I found when you stepped on it from a stop, there seemed to be a slight pause before it kicked in. On the highway it sounds like a pissed off sewing machine to me. Surprisingly, I found it the loudest. The interior was completely disappointing - an old tired look from 10 years ago.
Ford - loved the King Ranch. Very nice ride, the engine responded nicely. However I do not trust that diesel yet. Ford's 4th one in ten years or so. My next door neighbor is a senior executive with Ford. Even he advised me to stay away from their diesel!! Too many internal reports of engine problems that still exist. Even he said Ford should have gone to Cat or John Deere for an engine!
Ram - Loved the Laramie Longhorn. The Cummins was responsive and I found it quiet. It was the hardest riding of the three, but know that it has a light tire setting when not towing that improves the ride. The interior is simply amazing and has left the other two brands behind - they have catching up to do. I will also say I am VERY impressed with the mileage. My average is 29.5 mpg on the highway, not towing.
In the end I went to the 2013 Ram, 2500 Laramie Longhorn. This was a huge leap for me having owned nothing but Chev's all my life.
Good luck on your decision!