You might be really surprised at real world results with the V6. Its been my experience that the smaller engine on a larger truck is always working a little bit harder than a V8 would. Especially towing, that V6 is going to be working harder than the V8. That harder work will equate to real world mileage being roughly the same.
If the V8 is getting 18 mpg (which is attainable), then the V6 at 4% better would be 18.7 MPGs. Now if you are towing a lot, that V6 is probably going to be getting 7-8 MPG (estimate) and the V8 would be getting more like 10-11 (estimate).
I would seek out real world numbers from people and make an educated decision armed with those facts.
Honestly, for a minimal (and less than 1mpg is minimal) loss in unloaded mileage, you are getting a HUGE bump in power when towing/hauling. It was a no-brainer for me going from the 4.6 V8 to the 5.7 V8. I leave 1 or 2 MPG on the table with the big V8, but man does that big V8 produce some power!!! I can always drive more gently to squeeze out a little better mileage... its a LOT harder to squeeze out an extra 80hp...
