Thank you everyone, looks like we've narrowed our choices down to Westin Chosun or Sheraton D Cube. We are newbies to Seoul. Will do some research and reviews and go from there. Definitely will look into Subway access as well.

Don't have any SPG status, not sure if they match Hilton Honors Gold or Club Carlson Silver (but I will look into it)

Really appreciate everyone's comments.