The GC is also a unibody, so there isn't a frame/durability/towing strength difference there.

You can run the numbers analysis all day long. Before I bought my 35d, I analyzed everything you did as well, and included the GC in my analysis. I also looked at Audi, Lexus, Mercedes, Toyota, Porsche, Acura, Honda, GMC, Volkswagen, and a passel of used cars. I ended up finding that there were 6-7 different vehicles that 'checked all of the boxes' for capability, fuel economy, seating, towing, and objective criteria. Then, I drove all of them, and couldn't bring myself to buy anything other than the X5.

As far as towing with the diesel, it will to anything that you'd feel comfortable pulling with this short of a wheelbase. Grunt is never the issue. She's nice and stable for such a short SUV, but you won't confuse it with a long wheelbase, extended cab pickup.