As you can see, many different opinions on the 3.21 to 3.92 question. Here's my take, I have a 7k GVWR, 27' TT that I pull with the 3.21. I didn't order my truck but highly considered the 2 options when looking at dealer lots.
I think the biggest factor is how often do you actually pull your trailer. I only use mine 3, maybe 4 weeks out of the year, but the truck is my main transportation the rest of the year. I just pulled my trailer to Yellowstone last August about 3k miles. I had no issues with pulling the trailer in the mountains from a power or gearing standpoint. The problem of limited gas stations in the west may be a kind of headache, so the larger 33gal gas tank may be worthwhile, but also taking away from the towing capacity for the added weight.
I am happy with my decision especially with the gas price going crazy. As said in one of the posts, my truck gets 20-22 mpg on all highway in the summer, and about 17-18 mpg mixed local/highway. Winter gas cuts down on mileage, and I also found on my Yellowstone trip with the trailer, non-ethanol, high octane fuel got me an extra 1-2 mph with the trailer. If you were pulling the trailer a lot more often, especially in the mountains, maybe the 3.92 would be the better option, but that's something you have to decide.