Just went through this last month. Basically, I wanted 07+ suburban / yukon xl. I also wanted DVD player(s) and 2nd row buckets for the kids. Other than that, anything was in play.
I found out that the 5.3 in the 07+ has the 4 cylinder shut off, and that can cause oil burning in older engines - so I wanted a decent warranty to cover that. In the 6.2, that was added in '10.
I found out that there was not much difference in price stepping up to the LTZ trim, on a gently used vehicle. But, there was a bump going up to the denali. For my purposes (wife running kids around, grocery getting, towing a sailboat on the weekends, family trips) I didn't need the 6.2 - I basically really liked the sound of it.
After a good amount of test driving, I decided on a 09 Suburban LTZ - certified used, with less than 40k on the clock. It had just gone through the certification, got new brakes, fluids, tires and emblems. Part of the GM certified program gives you 12 months / 12k miles bumper-to-bumper warranty, to get the little things taken care of. It also gives you 2 years of free maintenance. The dealer I use also throws in a lifetime engine and drivetrain warranty (nice!) so the oil burning and transmission questions were answered.
We're really happy with the outcome. My advice is to drive a lot of them, even new ones, so you can get a feel for how they are driving (whether they were beat on in their prior life) and be aware of the certified program. good luck!