Has virtually nothing to do with the engine/power and everything to do with the class of truck and tires/wheels/suspension.
An 8.1 would not even be a good consideration imo unless you needed the extra power to tow. And at that, most of the advantage would be in the Alli vs the 4L80 behind the 6.0.
Toss up really dead reliable 6.0 with not as awesome of a trans vs 8.1 which could be great, could suck oil but the best trans.
And going old, it’s less about brand and more about miles and condition.
I’d be looking at older Super duty gassers equally. And 3rd gen Dodges if you find a cherry one. They’d be last on the list for gassers IMO.