Someone asked about the 2500 version so I'll chime in with a bit more info. The light duty 900 SUV's use the same chassis components as the LD 900 pickups, excepting coil rear vs. leaf spring.
The 2500 version SUV's use the same chassis as the 2500HD GMT 800 trucks, going back to ~2000, and 900 HD trucks up through 2010. That chassis was used right to the end of the 2500 Surbuban and Yukon XL, including the 8 on 6.5" bolt pattern, while the 900 HD trucks got a major chassis update in 2011 and went to larger everything, including bolt pattern.
I saw a youtube video that suggested the 6L80e used in the LD trucks was only good for 150k or so before having issues. I wonder how much of that is due to total lack of maintenance that most light duty vehicles are subject to... I can tell you that the 6L90e is mostly the same trans with some HD updates, and the one in the 2009 2500 we picked up recently works flawlessly after 200k miles of being used by a survey crew...
I too would recommend getting rid of the DOD if you get a LD truck. Poor man's way to turn it off is to run tow haul mode... That said, my dad's '08 has it with a couple hundred thousand miles and no issues. This again causes me to wonder if it has to do with poor maintenance practices, or a driving schedule that runs it more often...
I personally like the GMT800 trucks. They just have more style to my eye. The GMT900's went to an "every knob looks the same" interior, and I can hardly stand it. I don't care for the lack of body lines either. If you live in a rust belt, the 900's don't really have rocker panels, so you can run a more rusty truck that still looks nice if you keep the doors from rotting off... I also like the GMT900 seats, which are more firm than the seats in my GMT800, and the ease of adding the DIC buttons to a base truck to get actual TPMS readings and other stuff. I don't care for auto climate controls, and both our trucks luckily have the manual version. The GMT800 has dual zone controls, while the GMT900 has only one zone. Mostly an issue if my wife is along, as she is always cold and dual zone control let us both be comfortable...
Finally, the 6 speed is something I would DEFINITELY look for over the 4 speed. It just works so much better. Our '09 has 3.73 gears with the 6 speed, and it's far better than the '02 we have with 4.10 gears. More gears = more better IMO. Plus first and reverse are pretty low in the 6 speed, so it really gets off the link quickly. Also makes low range almost un-necessary.
Good luck!!