I think some folk need an introduction to high pressure common rail w/ pilot injection.
Chevrolet and GMC discontinued the stick shift Duramax 6.6L powertrains long, long ago. And when they were around, the manual trans engines were derated to lower torque and hp. The 5 and 6 speed Allison transmissions could take way more abuse than the manual trans or clutch could. It is not unusual for an Allison to over 100,000 miles towing at stock HP levels if you replace the spin-on filter and change the oil as recommended. I think at least one guy went over 250,000 miles IIRC.One thing I have learned is to never own a diesel automatic.
You ruin the diesel unless you run it through a good old 5 or 6 mt
New diesel chevies come with a stick for a reason, don't waste your time learning the reason through experience.
Just buy the manual off the bat and smile