i had a 2014 Hemi rcsb with the 8spd and got about 21-22 mpg in the truck with a Hwy/Cty driving, i have a 2013 rcsb 3.6l with the 8spd and i get 24-25mpg all day long and it gets higher on straight Hwy driving, that being said its only a couple miles per gallon, when it comes down to it, id buy a hemi just because its a hemi lol, the cant beat the sound of it, but the V6 has not left me wanting more in any way, it has plenty of power, alot of pep in it, and getting 24+ mpg out of a full size truck is pretty damn good too, i bought my 4th gen to be my work truck, i dont tow with it so i have not one complaint about it, if i was going to be towing on any kind of regular basis i would get a hemi but just for daily driving, you wont be unhappy with a v6....
ill add that my 3.6 has almost 160k on it and still gets great mileage and runs perfectly