A diesel will get better mileage while pulling, but remember that diesel will cost .50 to 1.00 more a gallon. So when you run the entire cost of a diesel its not as superior to a gas as it would first appear. That being said, if your pulling real heavy diesel is the only option. I consider gas engines only an option on lighter pulls. Also the new 6 speed trannys are much more effective at putting the power to the ground than the typical 4 speed. As the 8 speed and above trannys become mainstream it will make gas engines even more viable.
If I were buying new and I was routinely pulling around 12K and every so often around 15K I would go gas. At those weights you will never see the cost benefit of going diesel. If I were pulling 15K routinely then I think the scale goes the other way.
The cost of these new diesel trucks I think should cause some rethinking on when to check the oil burner box. It used to be you could buy a diesel simply because you wanted to, maybe some still can, however most of us need to be more careful. During the boom of the mid2000's I bought my wife a new 03 2500HD CC duramax to drive to work. She now drives a La Crosse. I would normally own a diesel even just to drive around to bid jobs an occassionaly pull something. I now own said 2500HD gas truck for such work, it gets 15.5 on the highway. My 06 3500 DRW Duramax gets 16.5. I paid the same for the 2012 truck as I did the 06 Duramax.