DH pm'd me a couple months ago. Derale does appear to be the better choice. Since then, I've decided to go another route to add hp and I have some advice to those that don't plan on going with huge power, and more importantly, for extended periods of time.
A shift kit should help as well as swapping your trans fluid a little more frequently than recommended by the fsm. Get any shit out of the way that might block flow in front of or behind the radiator. Whether you've got stock exhaust manifolds or headers, you can blanket them while on the FX using a couple brands, some better than others (search header blanket) which won't do shit for the trans but will keep heat of the engine and out of the bay. Also, shielding or wrapping the cats and exhaust pipes, at least what's near the transmission, will keep external radiant heat off the transmission thus providing some relief without an added external cooler.
Get a transmission temp sensor to monitor if you're still concerned and that will tell you if it would be wise to add an external cooler.