My view from both used in towing. So lots of miles, idling, and fleet work. The Ram 5500 is one of our twin line wreckers, the weakest link we have had with that truck has been the turbo, not sure exactly what the issue was but twice went in for turbo issues, the last time it went it was for transmission issues. But it is a beast and does not seem to hate idling long periods of time when dealing with difficult winch outs.
F550s we have are all 6.7, 2 wheel lifts and 1 rollback. The rollback got all the SCR sensors replaced and I think a fuel pump (2011 MY), other then that has not been in other then scheduled maintenance. The two wheel lifts I have not heard of them being down yet for anything more then maintenance.
YMMV since tow trucks get a lot of abuse.