Regardless of costs because let’s be honest, no one drops 70-90k on a truck to save on money, the diesel just makes that much more sense in these trucks than the 5.3 and even the 6.2.
In real world driving, the 3.0 is very zippy with massive amounts of low end torque. Compare that to the old dog that is the 5.3, you can step on it all day long it will barely move. While the 6.2 is slightly faster 0-60 and has the V8 soundtrack, it’s objectively worse at everything else. The diesel is heads down the better choice for this vehicle. From all the anecdata researching online there’s also nothing that makes me believe the 3.0 is less reliable than either the 5.3 or 6.2 either so long term costs are likely to be roughly same.
The main issue with the diesel is they are very few and far in between. Good luck finding one on the lot to test drive. If it wasn’t for the lack of availability and the bad (often unfounded) rep that diesels get, I’m certain majority of buyers would pick the diesel. IMO for the measly $900 GM should’ve made it the base engine and replace the outdated POS that is the 5.3.