Out of the major truck rental companies (Penske, Ryder, Enterprise, and the like) how do their largest trucks differ?

I have a business contract through Penske, and their 26 foot trucks have heavily derated engines. Even up a slight hill in Ohio, i can go from 70 (max governed speed) to 50 at the top. I called Penske, and they said the engines are derated for maintenance reasons. Now on a transcontinental drive, the mountains would take a toll on this truck, going up very slowly, and (with the exception of Penske's Hinos with exhaust brake) down the hills at considerable speed and brake wear. The question, does one company's trucks have more power than a compeditor? I know Ryder has dual fuel tanks, which are a plus, but i'm not in the mood for pure torture on even the smallest foothills, let alone the joys of driving an under-powered, under-braked truck down an eight mile 8 percent grade. Sure it can be done at a slow enough speed, but looking for the most reliable, highest spec-d truck available.

For the record, i have a Class A CDL, and in an extremely tight pinch can rent a tractor trailer.