I just purchased a Denali and looked hard at the slt vs Denali. I only went with the Denali because of the deal they gave me to purchase it.
The Denali with the ultimate package has the 6.2 l. Auto running boards. Moonroof, lane departure, break controller hand stiched leather hill decent along with the magnetic ride, 22 inch rims and the digital display.
The Denali without the ultimate package does not include the moonroof running boards the 22s the 6.2 or the magnetic ride control
The slt can have all of the above options with the exception the magnetic ride and the leather hand stitched seats and digital dash. I have never seen the auto running boards on an slt but they could be ordered as a specific option.
When I looked at an slt that had the 6.2 and the alert driver package it was within 1-2k of the base Denali.
I waited the dealership out (3 weeks) until they finally lowered the price on. Baseline Denali. Got it for 1000 more than the slt with the same options minus the couple mentioned.