I have owned both. As far as daily driving, the 5.4 will get you much better milage. When towing youll like the 7.5 better. Tons of torque. I would imagine that you will see about the same milage towing.
My 460 is a decent build and I haven't towed with it, but I know enough guys that have over the years.... Any gasser is going to see around 7-9 MPG with a loaded trailer behind it.
Deisal is the way to go if you do a lot of towing in my opinion....