Edit:

Checking out that website Between9and5, they also have contract with the Novotel Citygate in Tung Chung. Even though it cost more, you save train/bus fare, and more importantly, TIME. Unless you don't mind missing a couple hours of sleep to see the city.

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If you actually decide on Charterhouse (I have no opinion), it's really quite a walk from the train at Causeway Bay MTR, even if the Times Square Exit isn't far. I wouldn't do it, because you will have another fairly long transfer from Airport Express to the Island Line MTR already. If you take the Airport Express, just take a taxi from station to hotel; or take the free H2(Wan Chai & Admiralty) and get off at the Novotel. That's the closest stop, and if you just follow a map or GPS, it's 5 minutes walk for the 350m/yds.

Alternatively, Cityflyer A11 bus (every 20 minutes) has a stop at Tonnochy Road, which is even closer. But since you're doing in on same day, the same-day Airport Express roundtrip ticket ($100 to HK Station) is the best value, and not much more expensive than the bus.