My mind thinks of it like this.

Routing the cooler before the radiator, means lower trans fluid temps going into the radiator. Which would be less to drive up the overall temp of the radiator itself, thus lower coolant temps. This seems like it could also raise trans fluid temps that have already cooled by going through a hot radiator after the cooler.

Routing it after, would have lower incoming temps for the cooler itself because the fluid already traveled through the radiator. And most likely have lower trans fluid temps, with no effect on the radiator temp.