Diesel is the way to go...I know that the 6.0L gasser isn't the best on mileage. On the highway unloaded I can get 18-19 mpg and in town I get 15-17 mpg. Towing, I get about 12-14 depending on the time of the year. (winter diesel sucks) There is a greater repair cost with diesels, but the engine is covered by a 5yr/100,000 mile warranty that is a $100 deductable after the 3/36000 mile warranty is up. Now, it you are looking at the 07s, the warranty is 100,000 on the truck. Under normal use, the engine should be pretty much trouble free. The people that have problems with the engine are usually self inflicted. Once you tow with a diesel engine, you won't want to go back to gas.
i get around 17 mpg as an average but then again that was when both diesels where stock and with a exhaust and a good tune i increased 2 to 3 mpg an average around 19mpg on both trucks mostly highway with a small amount of city driving but when it is all city but wait lets not go there..i have seen up to 22.8 mpg but it only happens every now and then when i don't drive like i stole it..
ps i'll never go back to gassers unless i'm forced..
If it helps...I bought my truck strictly for the gas mileage. I live in the sticks where the roads do nothing but go up and down and curve all around. And driving around here I get 16.79 to 17.84 and when I went to South Carolina I got 21.36. As for towing...I've only towed a couple things so far...and I didn't even know they were there. But other things to take into consideration...Make sure you take care of your truck and change filters and grease it. If you do that it should last a LONG time. I know a guy who travels from Cali. to PA for a living...he has 539,000 miles on his truck and it runs like a champ.