Somewhere around 2010 the suburban came standard with the 6 speed vs the 4 speed. Much better for towing. For the record all the Yukon XLs Denali's have the 400 hp motor which is a monster, but a bit soft on suspension. IF you go with a normal suburban (may not work for you), a Denali is about the same price as a Suburban LTZ, but with the 6 speed wall the way back to 2008 with the big motor.
I second what others have said....take a good look at a crew cab truck. They can seat 6 if needed, only one person less than a Suburban with captains chairs.