I've got a comparison for you.
'01 HO 6spd, 4wd, QC, dually, 3.55, stock tires, Edge EZ on middle jumper. My dad's.
'05 Sig. Juice on level 1, stock tires.
They are damn close! I've pulled my 21' toyhauler (7k with bike + water, large frontal area for drag).
The '01 stays cooler on the hills (engine and EGT). Having the 6spd is perfect, running 60-65mph in 5th on a steep climb keeps all needles happy. 6th is a little too slow, lower boost and hotter temp but still safe and maintains speed.
Exact same hill with same load at same speed in the '05. 4th is on the verge of being too tall. It will pull it, it will accelerate on it, but the EGTs are high (thank you Edge). Drop to 3rd and now the engine is singing, more boost, slightly lower temps.
The '05 will pull harder (accelerate harder) with the load, but will usually run a little hotter while maintaining speed. If my '05 had a 6spd, then no contest the 3rd gen would win hands down. Don't get me wrong though, pop's '01 is no slouch and does really good when comparing these two trucks, simply because of the closer gear ratios!