Greetings!
I may be able to answer some of your concerns / questions. I had an '04 XL Denali that we loved, and it was hard to get rid of. When the GMT900's came out, I wasn't really sold on them, as the body style was "weird" to me. The more my wife and I looked at them, the more we liked them. The NBS really had to grow on me for several months. Finally, I saw a black '07 XL Denali sitting next to me in traffic one day, and I said "man, I gotta go drive one, that thing is wicked looking!". I
had been looking to replace my older daily driver, not my newer '04 XL Denali, which was in like-new condition with low miles and looked totally wicked (in my mind anyway)...
Well, going to drive a new one was what sealed the deal in my mind. The power, handling, overall performance, fit-and-finish, more organized controls and dash, and the upgraded technology (along with the convenience and safety features that made my wife and I both smile) where what sold us. For us, the '07's power fold-and-tumble second row seats and the power liftgate were to MAJOR improvements. I was
really on the fence about the decision (I mean, WHY replace a vehicle that there was nothing wrong with), but I landed a deal in my work that, uh, "sweetened the pot", so we went for it. The whole family likes it much better.
Problem-wise, the '04 front differential (AWD) went bad (under warranty), but I was a little surprised that happened at all, especially at like only 21K miles. The '07 has been fine (except for a minor temporary issue I had inadvertently caused with an intake installation, messed up a vacuum hose) though it's only got like 9K miles at present.
The '04 really was awesome, and I'd have loved to have both (couldn't afford it at the time), and I'd like to be able to replace my '97 Grand Caravan with an 02-05 Denali (non-XL) in the next year or so. SO, ironically, I plan to own both again!
Choose what fits your budget. The GMT800 and the GMT900 are both excellent vehicles, you'll really like either! (I even teared up a bit when I drove off the lot in my new truck leaving the faithful '04 behind...)