eh?
Two schools of thought here:
-More cooling is always better.
-Oil works best after it has warmed up, but isn't too hot.
If you prescribe to the first theory, then go with just an external cooler.
If you want you oil to warm-up, the transmission cooler actually serves two purposes. Initially it heats the oil up to just less than engine coolant temp, then as the oil temp rises it maintains it just above coolant temp. If it isn't enough, say because you are pulling a trailer, you would then add a cooler before the radiator one, so the supply oil to the transmission is always engine coolant temp. You gould get the same effect by using a thermostaticly controlled oil cooler by itself, but they aren't super cheap.
The same actually applies to engine oil. The built-in cooler in an automatic transmission radiator can work very well for maintaining engine oil temerature. I've used that trick countless times, and would highly recommend it to anyone.