I had a tech at PATC tell me that the radiator trans cooler dissipates heat 3X faster than an external cooler. Trans fluid also flows best between 175 and 225. With that said there's no reason not to use the radiator trans cooler. If you're towing or you live somewhere hilly, an external cooler isn't going to hurt anything.
An oil-to-coolant cooler is WAY more efficient than an oil-to-air cooler. Just engineering basics says that. You could plumb both coolers in series, which would be a little more insurance that your tranny doesn't overheat.
