@Benno.... My Colorado is an auto and I wouldn't have the manual for quids. For a start, according to the tech specs you get more "torques" as Jeremy Clarkson would say. The manual isn't rated to 500Nm. Secondly, the auto far more compliant and pleasant to drive around town and highway without the van. And with the 6 gears it seems ratio'd pretty well. And last of all when towing, I start off in manual and then when up to speed I slot it across to manual mode and simply manually shift from 5 to 4 when required. Trying to recall if I've ever shifted back to 3... Don't believe so.
So for me mate, it'd be auto all the way.
In respect to the trans-cooler, I was explicitly told that they didn't need one. I'm sure it can't hurt though, but I can't monitor trans temp (yet) so I happily live in ignorant bliss at the moment...