You still will get 400,000kms with a Cummins.
Youll get that with a duramax as well. Just comes down to how it's used and maintained. I have a dodge, it's a piece of crap compared to my duramax. Rusted apart already and the front end is falling out of it.
Duramax is designed for 200,000 hard commercial miles. Atleast you'll have a truck left when the engine needs a rebuild. You might have a cummins left but no truck.