The gas engine mated to the 6 speed transmission will have plenty of power at even the highest altitudes. We have had no difficulties driving at what ever speed we want in the Beartooth Mountains and many other high altitude locations. Keep in mind the gas engine is rated to pull a 15,000 pound trailer so unless you are also pulling a really heavy boat the power train will have a large reserve. Just be aware it's an overhead cam engine and let it spin.
Diesel is a great engine if you want it but as you noted you do lose a lot of payload and will weigh more with all of the implications (longer stopping distance, emergency handling etc.). There not the old breed diesels which could run any fuel etc. With all the pollution equipment they are now very complicated. The power and torque are impressive though.
I would think hard on the regular cab. You gain a fair amount of usability just going to the super cab.
