I had only been Seoul once and that was 17 yrs ago, so, I cannot comment which side has better view. But I do have some suggestions as general:

(1) See the view from the plane depends on which direction the plane approach the runway, isn't it? (eg HKG, sometimes approach from HK Island side, sometimes from Macau side, depending on the wind direction...)

(2) For me, if I fly in early morning, it means the sunlight can be very bright if you would like to keep window open. So, I usually choose the opposite side so that I can enjoy some view without sunlight burning me.

Have a safe & pleasant trip!!