If mileage is your concern here is food for thought. My F250 PSD gets the same mileage as my last 1500 GMC with the 5.3 V8. I average 15.6 daily driving (mix of highway and city) and get 18.5 to 19 on the highway running 75. Diesel is more than gas, about 30 cents a gallon here, and that equals about $6 everytime I fill up with 20 gallons. Those are the main reasons I went with a 3/4 ton diesel instead of a 1/2 ton gas when I bought this truck. We had planned on buying a 5er to, that played into it as well.
I would at least suggest you price the larger truck with the options you want before you buy. For me the 1/2 ton gas 4WD was around $2000 less than the 3/4 ton diesel 4WD. I was comparing GMC 1/2 tons to Ford 3/4 tons, so that had a little to do with it.
Just something to think about. It's a lot easier to buy big enough the first time than upgrade a year or so later.
P.S. The diesel blows the 1/2 ton out of the water pulling anything. I get 14.5 highway pulling a 21 foot bass boat 75 down the highway and the 1/2 ton got 10 on a good day.