The Gm duramax is an OK engine, just not to effecient. They usually need a programmer and some other upgrades.
Im a cummins fan but the new cummins isnt worth much IMO. It has decent power but crappy mileage. However, unlike new FORD and Chevys, it doesnt require DPF fluid. But im sure its coming.
Each make has its own issues, my advice, decide what you like and go from there. Sounds like there are a lot of guys with experience on here to help you steer clear of the problem children.