First let me say that I only ordered a Duramax at this point, but I did a lot of research prior to making my decision, about 6 months worth. I have driven the 8.1, 6.6 and Ford 6.0 only during test drives. Since the Dodge was 235HP, and no clubcab I excluded it.
Well, the 8.1 can come with an Allison as a option.
Seriously, you need to look at how many miles you are going to use it. For a typical useage the breakeven point is about 100,000mi. In other words, the fuel savings you will get with the diesel will pay for the cost of the option.
So obviously if you don't intend on keeping it 100,000, you're not saving any money going diesel.
There are good and bad things for both motors:
8.1 - Powerful, torquey. Enough power for nearly any legal application.
Low initial cost.
Lower maintenance costs.
Lower "per gallon" fuel cost, and fuel is everywhere.
Loss of power at high altitudes.
Expect 1/3 worse mileage.
Expensive and difficult to add significant additional power to.
Less fuel range.
6.6 - Plenty of power. Ditto.
High initial cost, but with the Power Package $1000 discount, it is the less expensive than the Ford. $3200 more than the 8.1/Allison combo.
Higher maintenance costs. But not by a huge amount. Neither are expensive to maintain.
Often diesel is more a gallon than the Regular, and less than Premium. Fewer stations carry it than gas.
Less power loss at altitude.
Good mileage for a heavy pickup.
Adding power is relatively simple and inexpensive.
Longer fuel range.
Weighs more, reduces payload a bit.
100,000 mile warranty.
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For me I finally decided to try Diesel Power. I keep trucks about 6-7 years, and put 100-200k on them. So I shouldn't lose money on the deal. And I really want a 400HP or more pickup yet still get good mileage. You can't do that with a gas motor.