Mileage - low mpg & regular gas before vs marginally better mpg & premium after - might be a wash in terms of cost. Driving style has more of an effect than most think.
For passive safety, the E53 is one of the best ever tested by IIHS. The big difference you'll notice is active safety - the X will handle better and certainly stop shorter than the Denali. I miss my Tahoe until I remember how wallowy and out-of-control it felt on the highway, and how laughably pitiful the brakes were. The X, in contrast, is rock solid at super-legal speeds
