The Ram 3500 is a truck built around one of the best diesel engines ever made, wrapped in a body that can't quite match it. The Cummins 6.7L will run to half a million miles with religious maintenance, but electrical gremlins and corrosion show up embarrassingly early, one owner found corroded connectors at 2,500 miles. Spec the Aisin transmission and budget for DEF system upkeep, and this truck will haul 36,000 pounds until the sun burns out. Buy it if you need maximum capability and can wrench or afford a good independent diesel shop; walk if you expect Toyota-grade fit-and-finish or can't stomach chasing electrical faults.
The Tundra is a tale of two engines: the 2007-2021 models with the 5.7L V8 are legitimately bulletproof workhorses that justify every ounce of Toyota's reputation, while the 2022+ twin-turbo V6 has suffered catastrophic bearing failures requiring full engine replacements on over 130,000 trucks, some grenading at highway speed. Toyota is replacing engines under warranty and extending coverage, but you're paying F-150 Platinum money for a truck currently in the shop longer than competitors and delivering worse real-world fuel economy than promised. Buy a late second-gen V8 if you want the Tundra everyone actually recommends, or wait a model year to see if the third-gen sorts itself out.