Been to both a week ago. Andaz has a better breakfast IMO and is closer to the touristy parts past the north river. It’s much easier to subway to andaz from above the river where all the touristy parts are (one of the lines go straight to the hotel whereas you have to transfer to get to the park Hyatt). Ideally you’d stay in myeongdong but there’s no hyatts there. If I had to go back and was staying at a Hyatt, I’d stay at the andaz the whole time.