if you want to take airport express train to the city center, you can choose Express Travel Pass. It includes single or double trip between airport and city center and metro (except some expensive metro lines) without limitations within 3 days, but can not use in city bus. Octopus card can be used in metro lines, city bus and stores, which is more conveninet I think. You can use it to take airport bus to the city center. It will be much cheaper.