Our current truck is a 2011 F250 6.2L gas, 6spd auto 4X4. Our trailer and gear weigh in around the 8,000lb mark. As we live just outside Vancouver BC just about every trip outside the valley involves the Coquihalla Hwy. For those that have never traveled it, it is a long climb that will put your TV to the test, and has claimed many. The 6.2l truck tows it beautifully and I haven't regretted the decision to buy it at all.
I have towed with a 2500 RAM Cummins 5.9 auto 4x4, and as a direct comparison I would say that the 6.2l gas (although completely different to tow with) gives the 5.9l a run for it's money. Maintenance costs were definitely higher on the RAM truck but the 5.9l was pretty much bulletproof (just loud), and as to be expected was more economical on fuel.
I should also mention that we have also owned an 06 Ram 2500 5.7l gas auto 4X4, and although it was always reliable and had enough power, the 4spd trans was terrible and the fuel consumption was by far the worst of any vehicle I have ever owned.... There is no comparison between it and our current Ford.
The newer gas trucks are definitely much more refined than they used to be..