I'll throw in one more vote for the Duramax after 10k miles.
I'm in SoCal and my wife daily drives it as our family hauler. She rarely drives more than 8 mi trips and its been great. We had one issue in the first 1,500 mi where a hose clamp was loose causing an emissions error code but was fixed same day and no issues since.
This was my first diesel and while I steeled myself the challenge, I worried about any complications that cause issues with regen issues or cold weather when we go skiing. Thankfully, all that worry was for naught and I haven't dealt with anything other than normal DEF fill ups (which are super easy). Obviously in the wrong environment, ie short drives in a Minnesota winter, you may fare differently, but outside of extremes, it truly is a great daily driver.
We love the mpg as well. We only fill up every two weeks and living in a hilly area of OC we average about 20ish mi gallon on short trips. Took a long trip to Santa Barbara with stop and go traffic through LA and averaged 27.8/mpg.
On performance, the car has never felt slow. Despite the fact its a 6,000lb beast, the low end torque gives it a quick jump off the line (about the same torque as the 6.2). Passing someone at freeway speeds is no sweat. Unless I was trying to 1/4 mi someone for pink slips, I have never felt there wasn't more power readily available. No doubt the 6.2 has "more power" in real world I think you're splitting hairs you won't notice.
Last thing I'll add is I was in the same boat, battling with my head and my heart, wanting the Denali. When we got ours a year ago the Denali's were still easily seeing a strong $15k+ dealer premium. Reluctantly I followed my head and got the loaded SLT and have no regrets. Yes there are a couple things that would be nice to have but that's life. Now, whenever we load up the family to drive somewhere and everyone is comfortable, the car drives great and I glance down and see the gauge that shows "25 mpg" all I can think about is how perfect it is and I couldn't be happier.
