The 6.7s have been out long enough that the major show-stopping bugs are out of them. I prefer the 7.3 because it is simpler, and has less EPA-mandated stuff that will break or result in a no-start condition, but the 6.7 isn't a slouch.
Call me weird. If I had to go for an engine for a recently made Ford SD truck, I would go with the V-10 gasser. Just because that engine is so relatively hard to kill, and works well for tasks that don't need a diesel. Plus, you don't have to wait for the cellphone yappers to put their device away, stop eating lunch and scoot their Prius out of the way at the edge pumps (the only ones with diesel) at the gas stations.