Hi Henry,

Better have an octopus card. With this card, you can get more benefits.
And I suggest you rent an on-loan Octopus Card at the Hong Kong Airport Express station or any other metro station. Very easy! Usually you need to pay 150 Hong Kong dollars including 50 deposits. It is refundable. You can get back your deposit and remaining value before you leave Hong Kong. However, 7 Hong Kong dollars will be deducted if you return it within three months.