Fuel costs while towing may be high in the 6.4L, but relatively even while empty. Make sure you get the lower rear end with the 6.4L.
Two comparisons.....
My cousin has a 5.7L 2500 towing 7k lbs TT and has never once complained. He has the 4:10 rear.
My father has an F250 with the 6.2L (very comparable to the 6.4). He tows a fifth wheel that's probably close to 10k loaded. He drove cross country, PA to CA and said he only wished for more power at higher elevations. He did say a diesel would make the trip easier.
If you're not towing all that much, go for the 6.4L. It's a really great engine, will have plenty of power, and you'll have a nicer truck.