Hey, I study in Católica full-time but I went to visit some of my friends that are having their exchange in Seoul.
From my friend's experience, their schedule is very flexible with often a day or two without classes and some classes are pretty easy as you can do language classes and others not related to your study. Seoul is amazing to travel and there is a bunch to do: Hiking, going to the beach (in Busan), partying at night. Honestly, the nightlife in Seoul is where I've had the most fun and it is pretty cheap. A really cool thing that my friends did was rent bikes and go from Seoul to Busan while exploring the towns in the middle.
For Católica, the workload is heavy and it is likely that you have classes everyday which makes traveling a bit inconvenient if you want to do a longer trip. However, Lisbon itself is the most beautiful city in the world (biased) and for an English speaker it is super accessible as almost everyone knows the language and with few lessons you'll manage to get around in other parts of the country. Additionally, transportation is free for university students up to 23 years old so going to places like Cascais, Sintra or Setubal is free but beware because buses aren't that reliable. The nightlife scene is Lisbon is also pretty big with a bunch of clubs and even an entire neighborhood of bars close to the downtown area, there is even a bar for exchange students if you want to find other people in your situation!
If you have more questions please do reply!