I know you excluded the Power Wagon, but my '13 PW is not getting that bad of mpg. 12-14. My '07 1500 4x4 with the 5.7 is only getting 3 mpg across the board better, and that is with the cylinder deactivation, a smaller, lighter cab, and 3.92 gears, rather than the PW's 4.56's. And I know the PW weighs quite a bit more, with the larger brakes, tires, sturdier drivetrain and winch, etc.
Now, if the new 6.4 with 4.10s and the addition of the cylinder deactivation can get the same, if not better, why not? You can never get enough bottom end torque. But, like it was said, it takes A LOT of miles to make up for the premium paid for a first year, new engine offering. I can't say I have any real complaints about the 5.7.
Sure, the highway cruising mpg difference seems to be a lot 17 in the 1500 vs the 14 in the 2500 PW, but at the end of the day, the PW is such a much nicer truck, I gladly will pay the extra 3 mpg.
And, the PW is in a class all by itself. Sorry guys, but even the gas 2500's, ride like covered wagons compared to the Power Wagon. Not sure what magic they did, but it really did capture the best of all worlds; It can handle the higher tow / haul loads and off road abuse with the solid axle, yet rides dare I say nicer, than my '07 1500 4x4 with the IRS empty or otherwise. Plus, it is just a much more solid vehicle.
Biggest trick; keep it at 70 or below. Both trucks really nose-over in fuel consumption at that point. The difference between running 70 or 75 will more than make up for the additional fuel consumed by a 2500. Both my trucks are bone stock, and run factory optioned tow package gears and large tires. A completely stripped down, standard cab 2014 model 2WD 1500 with "freeway gears" and the 5.7 Hemi may be able to add a few mpg to either of those, while still doing 80 on level ground. But, what fun is that.
Life is short. Pay for what you want and get what works for you.
Just my opinion.
Good luck!