Neither location is wrong.
In the line leaving the trans going to the cooler you will see higher temps as the fluid has gone through the trans. You may see sudden and often frightening spikes. Often times even a perfectly healthy transmission will see fluid temps going to the cooler on the high side of the acceptable range in a load situation. It's momentary but freaks people out sometimes.
In the pan you'll see temps after they've gone through the cooler. Rather than seeing what is going out of the trans you see what is going in. Temps are generally lower and fluctuate more slowly vs the cooler line. Vehicles that come factory with a guage or monitor read from the sender inside the pan
Myself, i prefer the pan. Im of the opinion that watching the heat soak of the fluid vs the temporary peaks are is in my best interest and it's often easier to do it this way. At the end of the day, both methods are perfectly fine. There is zero hard data that says one is better than the other but there is a ton of conjecture and opinion. This third paragraph is a great example of it
