The choice depends on circumstances. The new 2015 Powerstroke has twice the torque that the gasser has.
What it comes to -- what will the truck be used for? For example, I'm looking to buy almost exactly the same configuration of a Ford F-350 pretty soon... but since this is going to be a daily driver dealing with the city with the second worst congestion in the US (and a city council that seems to have little interest in anything changing that... except for adding tollways), I'm going with a gasser, as they are easier to deal with in a metro area.
However, in almost any other circumstance than mine, I'd would recommend the diesel. It costs $7-8k more, but it will definitely be worth it. The gasser will pull the trailer, the diesel will -happily- pull it. When going up steep grades, it doesn't lose power as normally aspirated engines do. Diesels require a bit more upkeep, and require DEF, but the different isn't that significant.
The other reason I'm getting a gasser is that I only have two fuels to worry about when boondocking. Gasoline (which will be used for both the second generator and the truck), and propane (which will be used by the TC's inbuild generator and the other appliances.) I normally carry about 8-10 gallons of gas for the generator, but when done and heading home, the fuel goes into the truck's tank.
To make a long story short, unless you have a specific need for a gasser, get the diesel. The newer Ford diesels don't have the bad reputation the "6.blow" does, and if maintained properly, will last a while.