I'm not sure how concerned OP is with resale if he keeps his trucks for ~20 years, but you're right.You might also think about resale value. A truck with the 8-speed will likely be worth more down the road.
You will still see an improvement in highway mileage with the 8 over the 6 despite having the same top gear. The 8 is much more efficient.I just finished pre-delivery inspections on a pair of Hemi Rams where the only difference besides paint color was one having a 6 speed and the other having an 8 speed. My take is that if most of your driving is on the highway, it really isn't going to make much difference in overall fuel economy since the OD ratios are the same. In city driving, I would give the advantage to the 8 speed. That being said, how many of you bought a Hemi Ram just for fuel economy?
Where did you get these? The 8 speed's reverse ratio is 3.30:1, there are no AWD Rams, and there is no different gearing in the trans for 4wd vs 2wd. If you get an 8 speed, your reverse gear is 3.3:1, even if you get the V6.8-Speed ratios:
R (RWD) 2.65:1
R (AWD) 3.06:1
OP, the 8 speed is also stronger, with a max torque rating of 516lb-ft vs 410lb-ft with the 6 speed. They could actually put the 8HP70 that goes behind the 5.7 Hemi in the 1500s behind the 6.4 Hemi in the HDs