Century Southern Tower is a bit closer to JR station but you need to walk across the street(with traffic lights) and walk in the open air for a few minutes before you enter the building and take a lift up to the lobby on 20th floor. but it is a very good hotel with nice breakfast too.
Keio is just a bit farther off, but there should be a subway (underground walkway) to reach it directly so that you don't need to walk in the open air for so long, correct me if i am wrong since i haven't been there for 2 years already.
so it really depends on the weather, if it is really cold and you've got a kid, you might consider Keio a better choice, but for the hotel, i like Southern Tower more.
Thank you for your advise. Actually I hv booked Keio Plaza throught Axxx room...It is HK$5580 for 4 nights, during 4 Apr - 8 Apr....seem very experence...but i hv 3 yrs old boy going with me so I don't hv so much choice....I want convienance so that I hv to pay loh! :-(