What is your max weight - trailer and load?
If under 8000 lbs, I wouldn't worry about the gear ratio and get the least rusty truck
FYI with the 8 speed - 8th gear with 3.92 is the same as 7th gear with the 3.21's and so on down to about 5th gear on the 3.21's. So you will just be in one gear lower with the 3.21's vs. the 3.92's when towing - when empty you will enjoy the superior highway mpg of the 3.21 gears if that is what you have. BOTH have enough power and lower gear ratio to pull from a start than a 3.92 geared 6 speed.
Also your speed limits are lower than ours south of the border so that will help also.