Out of the two, I prefer Discover University. You can already send your teens to high school. Going to high school with them can get repetitive after a bit. It's nice to have the option, but I find I don't want to accompany them everyday. I needed to add a mod to choose classmates to make this pack work for me. I'm a rotational player who plays in neighborhoods, so I wanted the teens in my neighborhood to be the ones who show up. Fortunately the mod works great for this! I have used the after school activities often.
I find I use Discover University often as it adds something not in the game at all. There are times as a rotational player that I only want the degree and the suggestion that someone has gone back to school to get it. So I install a mod that allows university to be completed with less credits. When I want to send a group to university after high school graduation, I remove the mod. I usually stay with the group until they finish university, but it's possible to rotate away after a term and just wait to re-enroll them when you rotate back. If you are trying to complete the academic aspiration, you'll want to give yourself the entire university experience.
As for which to buy, it depends on what will add the most to your game. What story do you want to tell?
