The Cummins 350 in a 2003 model year coach would likely be an ISC series (Electronic engine) and is an extremely reliable series of Cummins engine. When I bought mine, I called Cummins with the engine serial number to check if there had been any known issues or recalls. There had not been. In conversation with them, the person at Cummins said that they generally don't consider that engine fully broken in until around 140K miles.
I can't rightfully comment on the CAT. While I've driven MH's with CAT engine, I've never owned one.
Most importantly, it boils down to how well the motor has been cared for by the the previous owner(s). If properly maintained, I don't think that you could go wrong with either.