In my semi professional opinion, if you want a 5er, you want a 1 ton.
Dual rear wheel buys you on average an extra 2000lb payload (pin weight) and a touch more stability in crosswinds, but if the pin weighs less than 2,000lb, you really dont need to be buying an extra pair of tires.
As for the gas / diesel debate, i'm both feet solidly in the diesel camp. I love the torque, and how effortless it yanks 14,000lb.
Now for reality.
If you're staying semi local (less than 1000mi), or towing less than 6 times a year, go with gas.
It will burn more fuel, but the repairs and maintenance are significantly less (for example, my 6.4 diesel takes 14L of synthetic oil, at $160 a pail), and thats not considering the additional purchase price, warming it up and cooling down the turbos.
If you plan on doing mountains, or towing more than 6 times a year, the added expense and cost of repair is more than offset by the ease and confidence the added power gives you.
2008 F350 King Ranch, 6.4 full delete, studded and tuned. On board air, automatic air levelling