The diesel is great for towing & doing "truck stuff". For driving around town, not sure if there is a difference (other than it sure sounds cool). I do get around 15-16mpg w/the boat on the highway at speeds higher than I should mention. I think that would be better w/smaller tires (bought used w/315's, will go to 295 or 305's when I replace).
Will diesel continue to stay higher than gas is another good question. Historically, I think it is usually less. Also, you will get a lot more on resale. Look at 2 or 7 year old trucks for example...
We went with the diesel because I felt the '97 351 may be underpowered for a truck of this size, and I didn't want to pour gas into the big block 460. I do think the newer ford 5.4's have good power to them for pulling, especially if all your are towing is a 3500 pound boat. However, if you add 4 people, firewood, coolers, camping stuff, gas cans & who knows what else, the game can change.
Have fun shopping. One important thing is I think the F150 4 door only has the silly little 5 foot (or whatever) bed. To at least get to the 6-1/2 foot range, you need the 250. I think Dodge is onto something there, I have seen 1/2 ton 4 doors w/8' beds. Personal preference maybe, but when I use the truck, I like to have the space.