Get the V8, if you haven't already. Last month, I traded in my '06 Pathfinder V6 with 70k miles. For 3 of the 4 years, I towed a 6x12 tandem-axle, enclosed trailer twice a week with min. combined weight of 2500 lbs. Also, did round trips from VA to FL & from VA to TX, both trips towing my Z32 on a u-haul flat trailer (5000lbs). I beat the hell out of the '06. Just changed fluids, tires, did air filter maintenance & used premium fuel. Original brakes, battery, everything else.
I averaged 16.5 MPG in 4 years with the V6. K&N drop-in filter was the only mod.
The new V8 averages 15mpg with a K&N cold air intake & Jet maf sensor. I've only 1800 miles or so (est. 70% of that is hwy), but I've towed my 6x12 enclosed trailer 3-4 times (40 mile trips) and I'm pretty heavy footed. I still have some breaking-in to do. Also, that break-in fluid can't be good for mpg's so I may see some benefit.
I am very pleased with the towing capabilities of the V8. I will sometimes have an additional 2 tons of debris in my trailer, and although the V6 could handle it, I would struggle on hills and when passing. If you don't do that kind of towing, then the V6 will be more than enough for you. I wanted the extra power but the deciding factor for me was the price. I wanted an LE with NAV which was standard on the V8, but not the V6 so the pricing was comparable after discounts.