Right. This depends on the game....
I personally install older gen games on my HDD, but for open world games...Games that have to load often and load in objects etc. I install on my SSD. Some games performance will significantly increase on a SSD. So general rule would be....Demanding games on the SSD, and the less demanding games on your HDD. The game you play the most often would be ideal on the SSD too, since loading times will be so much better.
Simply put, you install games you want to run slighty better on your SSD haha.