I prefer the sportier athletic look of the AT4 so thats what I bought.
One thing I noticed right away was the suspension was a lot bouncier than my previous truck, an older sierra 1500. Of course, thats because of the off-road suspension that the AT4 was designed with
I think another difference you will notice is the AT4 is a couple more inches higher to climb into. Which might be a concern for older people.
Otherwise, I think the trucks are virtually the same, so its a matter of personal choice.
I think if I hadn't gone for the AT4, I would have considered Elevation. Sure its a lower trim level, but GMC did a really good job on the appearance of the truck and the value of the packages included.