6.6 gas over diesel for sure if you don't plan to tow more than a few tons regularly. The 6.6 is incomparable to the 6.0 that preceded it. The new 6.6 gas has direct injection, may need more frequent oil changes, etc, and the 6 speed auto is a proven design. In back to back testing with a Ford 7.3 gas with the 10 speed both trucks were nearly identical in MPG under the same load.
For me, I have clients that are close to one another for regular weekly service. On one block I have 4, and another block I have 3, etc. The truck is only running for a minute or less at times which isn't good for any engine, and especially bad for a diesel, so gas engine is the best option for me in terms of long term ownership/durability.
The Ford 6.2 is nearing the end of it's lifecycle for emissions, but is a solid engine and comes with a 10 speed auto in the F350, but only a 6 speed in the F250. I have a Ram now but will probably go GM next. HEMI maintenance is the highest of all the gas V8's. Copper core spark plugs that need replacing every 40k - 16 spark plugs! I was lucky to have an extended warranty as I had problems every other year with the exhaust manifolds coming loose from the block which saved $1600 each time thanks to the warranty. Only have 178k kms on the truck now and it's ten years old. Had a head gasket done at 168k kms and extended warranty covered that at $2500.08 CAD. Reliability is one thing, down time is another. I'm religious with fluid change times and the like.