I just came back from Tokyo and I stayed at Keio Plaza. I consider the hotel quite good. Room size is normal according to our standard (but big according to Japanese standard) and the service is quite good. In my 4 night trip to Japan, I stayed in Keio Plaza on the 1st, 2nd and 4th night. Maybe because the reception staff saw that I was carrying my baby, she was so considerate to give me a room with a massage chair! If you would go to visit Disneyland, there is free shuttle bus available from hotel door to Disneyland entrance.
I haven't tried Century Hyatt before but know that it is quite far away from Shinjuku station. Keio Plaza is about 8 mins from Shinjuku station. If you prefer a 5-star hotel closer to station, you may consider Century Southern Tower (just 3 mins from station) which was my first choice originally but due to full booking, I changed to Keio Plaza.