re: F 250 vs. F 350. What should I be looking for?

It takes X horsepower to move whatever at a given speed. Producing X horsepower at higher RPM is less torque and more revs. Less torque = less strain on driveline stuff. It's a higher cycle count which does wear out certain things faster like bearings, but for the most part it's much easier on everything to be higher in the powerband.

The torque curve is for wide open throttle. You're operating on a load curve, which is exponential but usually pretty close to linear where the vehicle actually operates.