As a Titanium member, I usually stick with Marriott's whenever I travel, especially in Seoul where there are multiple Courtyards. I usually prefer the Courtyard in Namdaemun as most of my stays are long term and that hotel has a laundry room in the basement. However, I usually stay at this property if I want to be near my family.
Differences: The Courtyard Times Square has larger rooms and a desk compared to the Namdaemun property, which is a positive. They also provide Platinums and higher their breakfast in the main restaurant versus the Club Lounge. The happy hour spread at Times Square is also better. Being connected to the mall helps when you want to grab some food but do not want to go outside.
My main complaint about this hotel is the HVAC system as the rooms tend to be very hot and you cannot adjust the temperature of the room. I was told that rooms facing Yeungdeungpo station tend to be hotter than rooms facing the fire station, which is why I switched rooms in the middle of my stay. This turned out to be true but it still was still too hot for my liking, so the staff brought over a fan to help cool down the room.
Aside from that, I would still stay at this hotel in the future if I need to be near my family. However, if the rate difference compared to the Fairfield is significant, I might give that place a shot.