Stiffer frames are not particularly a goal on trucks. Some stiffness ranges get you in real trouble with noise, vibration, and harshness (NVH) on things like standard road seam spacings and bumps. You don't want the system to be the same natural frequency as the input or it will get into resonance and bounce all over the place. If your system's natural frequency is lower than the input, it will attenuate the input signal (you won't feel the bumps as much). This is the entire concept of engine isolation/rubber mounts. That system has a very low natural frequency and the high frequency engine vibrations don't feed through to the chassis.
I'm not claiming I know any design goals for either truck, just throwing out some possibilities.
Also, normal automotive steels can range from 25ksi to 70ksi yield strength. We don't know what either is made from. Yield strength numbers (within one material) mean how far it can stretch before it is permanently deformed.
Kevin