there are actually 3 schools of thought on this.

first 2 mentioned.

before, so the rad will maintain a more consitant trans fluid temp.

after to cool a bit more than the rad can, as lower temps equal longer life.

third is to bypass the internal rad cooler all together.

A couple of aftermarket transmission companies recommend this option because of 1. the transmission can never cool further than your radiator temp using that internal cooler vs the external cooler that can go to ambient.

plus concerns for gunk in the cooler and some radiators are know for leakage.

I was always taught to do the first one. but after reading a few articles like that I will always get a bigger aux cooler and leave the radiator one out of the loop.