I've stayed in both hotels (Century Southern Tower & Keio Plaza) before for several times.
For Century Southern Tower, the rooms are a little bit smaller than the standard rooms at Keio Plaza. But its location is really good and, based my previous stays (a few years ago) the Airport Limosine (shuttle bus) do stop at the Hotel entrance but with less frequent when comparing with Keio Plaza. I think if you choose to stay at this hotel, it would be much faster to use the JR from Airport (rather than taking the shuttle bus due to the traffic jam).
For Keio Plaza, the airport shuttle schedule is much frequent and you can book the free shuttle bus service to and from Disneyland (simply via email to the hotel concierge).