I have had every different engine that Chrysler has put into a truck and here is my OPINION.
5.2L : Great Engine backed by a terrible Tranny.... Spark nock BAD 15mpg
5.9L : Gas GUZZLER.....Not enough power over 5.2L to Justify....
4.7L : Great little engine in my Dakota Quad Cab 18mpg....Durango 4.7 15mpgL
5.7L : With non MDS..... Averaged 14 MPG all the time....Powerful..Tranny Stunk
5.7L: With MDS..Highway with Summer Fuel = 18.5 Highway, and 15.7 Mixed
Winter Fuel = 16.7 mpg highway and 14.5 Mixed
The new 4.7L is all that most will ever need. 300+ HP is plenty. I have a heavy boat and the 5.7L is the smallest gas that I would want in front of it. The TQ Curve of the 5.7L is awesome. My Brother has a 2007 4.7L, 4X4, that he cannot get over 15 mpg on the highway no matter what he does. This is the lower output engine. The biggest desciding factor when buying a Truck is what will you need it for. Heavy hauling over 4K or just light duty adventures. If you want the HEMI then you will not be happy with the 4.7L everytime you crank up the truck. Drive Both..... The money saved on the 4.7L will be hard to make up with the slim margine of better MPG you will get from the HEMI.