The 2015 Denali will have the 6.2L where as the 2016 SLT will have the 5.3L.
2015 was the first year of that body style so it may, I say May, have a few bugs that may have been addressed for 2016. That being said, I would think after 143,000 miles, any first year issues would have been likely dealt with.
A lot of people prefer the bench seat for 2nd row, but that’s a personal preference.
What suspension does the SLT have? The Denali will likely come with premium suspension, that may have been optioned on the SLT.
At the end of the day, I don’t think you can go wrong with either one if everything else is equal.