Hi i'll be at HK from 21-28 and will use airport express to and from Kowloon.

The Airport Express Travel Pass with two single journeys is highly recommended. You can use it to ride on AE train. Also, you can use it to ride free on all MTR lines (except East Rail Line First Class, Lo Wu and Lok Ma Chau stations), Light Rail and MTR Bus (Northwest New Territories) within 3 consecutive days.
In addition, you can top it up, and use it as a normal Octopus card to ride on buses, taxis, ferries and trams on the rest of days' public transportation.