Been to both....i don't think they compare attraction wise to Universal and Disneyland but both parks have a sheer number of attractions to where you definitely won't be board and feel like you get your monies worth.
Lotte World mostly indoors with a very small outdoor section so I would vote for that especially in December unless you are very good at handling cold weather. The outdoor section has the more thrilling ride while the inside you have one coaster, a indiana jones/dinosaur knockoff ride, and a bunch of flats covering five floors. Lotte World gets ridiculous crowded on weekends so I highly advise you go to the park on a weekday.
Everland doesn't have anything Harry Potter related but it is quite large and spacious. For me Everland is Busch Gardens Tampa meets Busch Gardens Williamsburg. You have the heavily African/Jungle based safari world with too vehicle based animal seeing tours and a rapid rides followed by a large amount of areas based in Europe/America. Everland is a bit outside of Seoul so be prepared for a roughly 1 to 1 and 1/2 hour journey to Yongin via train and bus, then getting on train/bus to the parks. Also for everland, you will need to make sure you have your phone charged as you can only use your tickets via phone (This was as of Feb 2023 when I went last time).
Seoul has so many things to do and see and it really depends on your mood, age, mindset. There is a lot of tourist stuff but if you want to find all the real unique stuff, the locals are key and the best way to meet locals is through the nightlife. Go to the bars and just talk to people and they will be able to tell you all the best food and things.