Any port in a storm! Reducing operating temps by as little as 20 degrees will extend transmission and fluid life like you would not believe. When you tow, a biger surface area in a smaller package like a plate cooler really shines. Some of them even come with flat motor fans to further boost cooling in slow moving conditions and can be set to run with a thermostat too. But to verify what you have and plan to do - yes on both. Any little bit helps.
I am planning on a Jegg's kit come Spring/Summer. It's too dam cold to be putzing around with the truck these days, and no daylight after work. :P