Empty, the 4.6 will move the truck along, but in my experience the extra power of the 5.4 comes in very handy when towing. Its basically a preference. The 5.4 might get 1-1.5 mile to the gallon less when running empty, but towing the 5.4 will be the same or better. As for a diesel, unless you are going to get something heavier to pull or plan on pulling all the time, I can't see that as justified. A comparible F-250 diesel with that mileage will be atleast another 5-6K, probally closer to 7 or 8K. Not to mention diesel is $1.20 higher than gas here right now.