Either way, you're going to need to run the tranny cooler lines from the tranny, wherever you put it. The best place for a air to oil tranny cooler would be up front by or in front of the radiator in the airflow, but I suppose you could put it underneath, closer to the tranny than the radiator would be.
A tranny cooler (or transmission warmer as Mark says) in the radiator would be the most convenient thing to use as far as finding a place to mount things.
An OTW cooler in the radiator isn't a transmission warmer until the engine starts to overheat. Guys I knew who used to drag race Diesels with auto transmissions had temperature sensors in the transmission pan and would only get concerned if the temps got way over typical engine overheat temps, and that only happened after a few passes down the track. An OTW cooler would still cool the transmission, but would function as a warmer only if the radiator temp was warmer than the transmission as in first driving after completely warming the engine up.
