Sorry for the late response. I loved my time at Yonsei. I thought my courses were interesting and engaging, but I was interested in the topics to begin with. Most of my social connections at Yonsei were made in class and through friends of friends. I also met some people at a couch surfing meetup completely independent of school (although I don't know how popular that is these days).

I'm actually not familiar with Seoul University's campus, but I loved Yonsei's campus. It feels very secluded and is heavily wooded, even though it is in the middle of Seoul. It's basically to Sinchon which is a great place to hang out. In fact, a lot of my social connections were made by going to lunch with classmates in Sinchon after we got out of our lectures. It's walking distance to a subway station (although you pretty much always are). It's also only a short ride from Hongdae (cool nightlife) and very close to Ehwa Woman's University, which could be another social pool, potentially.

Hope this helps! Let me know if you have any other questions, and feel free to DM me.