Millenium Hilton--rooms are quite nice if you get club floor. Agree that lounge b'fast and canapes are good. To get to Seoul Stn, you need to go down/up fairly steep hill, takes about 5-10min walk. Actually there's another closer (and smaller and easier to navigate) subway station also at the bottom of the hill. I don't think their disco/casino is the hottest entertainment.

Park Hyatt--personally I'm not a big fan of Gannam esp if your main attractions (palaces, insadong) are north of the river. Though good subway access (again about 5-10min walk but at least it's flat) and MUCH easier to find non-hotel restaurants after dark. One major advantage of staying south of the river is it's an easier getaway to the airport.

If you really want 5*s in convenient location, Westin is in the middle of Myoeng Do.