I have lived in Korea for 15 of the last 25 or so years. The two above are good. I would also check out Platinum in Apgujeong. To my tongue, it is alot closer to US microbreweries in taste.
Craftworks has a good selection, but they all differ more in color than taste or mouthfeel. That is to say they are all a bit thin and watery. Good burgers and single malts, though
Another happy new development is select bars now stocking some good West Coast brews. Look for Rogue, Lost Coast, and Anderson Valley among others.
Finally, since you are there, try to get your hands on some good local stuff as well. Makkolli and other rice brews are making quite a comeback.