2720 vs. 3124kb??? reality of tow rating?

Much thanks for all the input! There are so many pieces to this puzzle, and I am trying to power through my learning curve here.

We do live in AZ, and I am hoping to drive through both mountains and flatlands... so. I will pick up the 2720, that we found a deal on. I will go ahead and buy my minivan, but hubby agreed to hang on to our ML350 SUV with a 7600 tow capacity so when we are going on extended trips, or tough terrain, we will have a more appropriate option.

Thanks again for ALL of your help- we'll be seeing you again, I am sure!