I can't believe it took 8 posts before the most logical answer came up
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- Airport Express to Kowloon and then free hotel shuttle buses. I live in TST Gateway and I ALWAYS take the hotel shuttle buses. From Kowloon station, it doesn't matter if you have no luggage - you just have to prove that you have taken the Airport Express, either by showing the Octopus Card or getting a slip of paper from the information desk as you disembark the train and before going through the turnstiles (once you do, you've lost the ticket - the machine keeps it).
I only take the taxi after 11 pm as the shuttle buses stops running then.