To compare my experience:
2021 burb 5.3 with 15000 miles when I traded it in was averaging 19.3 by computer (which I found generous compared by hand calculation)
2022 burb 3.0 with 10,500 on it 26.8. I’ve towed more with this rig. Average 21 when towing.
I also had poor luck with air ride suspension with GM. Ended up in shop for 2 months waiting for parts. The dealer thinks it is an O ring issue which is sandwiched between 2 washers. It was 40 degrees one day followed by -20 the next. Air ride went out likely due to temperature or potentially moisture in the system.
Do your own due diligence. There are horror stories from both sides. I find the diesel a nicer drive overall. When my current rig is done I will order a new one with a 3.0. In my opinion they are completely different vehicles in terms of how they drive. I prefer the diesel—even if it costs more to fuel and maintain.