As much as I generally speaking like diesels, I would get the V10. 13-15 empty is not uncommon. I have a friend with a 4 sp auto, and he gets 13-15 with it, loaded to 10K with construction tools etc.
The V10 is a bullet proof motor. The 6.0 main block is a good motor. but Ford did some other external stuff that really screwed the pooch with that motor. If you get a good one, you're lucky. Otherwise, it is like the old GM 350 diesel, or 6.5td, where they can be pricey to fix when things happen. At least with the GM models, fixing is cheap vs the 6.0!
marty