I think you have to ask yourself, what's more important?
More Horsepower and Torque or a little more fuel Mileage.
At 3 or 4 mpg gain at. How many mile do you have to drive in order to re-coop the cost of that tuner or chip?
During that time, it's gonna be hard NOT to play with that extra Horsepower = drop in mileage = a few steps back in re-coop cost of tuner.
Your driving habits and driving technique can also make a similar gain in fuel mileage
I didn't by my truck for it's fuel mileage capabilities. I love my truck for it's HP and Trq