Help me choose! Discover University or High School Years?

I think it depends on how you play the game. Do you like focusing on teens in high school, or do you focus more on adults? I do the latter, and I think UNI adds quite a bit to each student's personal development in order of choosing a degree, which results in choosing a focus in their lives, which again will give a few favored careers and probably a unique life based on all this. A high school student might have more fun during school years, but afterwards I don't think the sim will be much affected by those high school years. Like others said, the best go might be to get both packs ;)

I love both worlds, but I think Britechester is prettier and has more variety and options. Copperdale is more secluded and with a different backstory that also feels good.

University life is fun because you can set it up as you like, choosing how many classes to follow per term. I've not played HSY enough to compare.

I think UNI objects are good, but so are HSY. I use objects from both packs a lot. I appreciate CAS from UNI a bit more because they help me making YA sims look younger than Adults, and also my older teens will often use UNI CAS. HSY CAS is good too, but (I find it a bit too fashion-ish for my average teen. It might be better if you play super popular HS students, with an interest for the fashion feature that is included, but I never tried. BB from UNI is a bit more plain and simple, what you would expect from a not so exclusively equipped school (can also be good for low standard hotels, retirement homes, etc).

I never focused a lot on after class activities, but I'll admit the HSY activities might sound a bit more fletched out. I think UNI organizations can play a role in a sim's life also after graduating, but I'm not sure. 

It's a tough choice between the two. I prefer UNI but that might be because it os already well implemented with my game style, and I did not yet explore all sides of HSY.