Either one will do the job just fine. Each will respond differently for each driver so it is hard to say you or i will get xx mpgs.

I have the 6.4 and when I had my 8K tt I could achieve 9-10 mpgs driving 70 mph.

empty I get around 13 city and up around 17 hwy unloaded.

Flatbed trailers I can get around 11 loaded.