I know this is a late response but I figured it could help someone out in the future. I have the 3.92 and my dad has the 3.55. He can average about 24mpg with normal city/hwy mix driving while I do roughly 21mpg. I've driven his truck quite a bit and am able to get the same mpg in his truck that he does so I can conclude our driving habits are similar. As for towing capacity, all trucks are engineered with safety ratios in towing capacity. Just because it says 8.5k towing capacity does not mean it cannot do more. It is simply not recommended. However, safety factors often decrease load ratings quite a bit. Just cause you are near or over 1000 lbs of the stated tow capacity does not mean you are near an unsafe tow capacity.
We towed a small trailer with a 2wd frontier that was loaded with vinyl plank. This load was well over the towing capacity of the truck (it was a one time tow). The truck held up well even though it struggled slightly, in the end it was the trailer that ended up giving in haha (blowout).
My recommendation to anyone pondering the same question is to go with the 3.55. It is far more than what your typical loads require (boat, atvs, construction materials for DIY projects) and it is worth the better mpgs.