What octopus card should I get?

Well, one single journey ticket costs HKD 100 between HKIA and HK; HKD 90 between Kowloon and HKIA; HKD 80 between Tsingyi and HKIA. If you buy a round trip ticket, you just need to pay HKD 180 between HKIA and HK; HKD 160 between Kowloon and HKIA; HKD 110 between Tsingyi and HKIA.

The best choice for you is the Airport Express Travel Pass with two single journeys. It costs HKD 300, with a refundable deposit of HKD 50. In addition, you can take all MTR lines (except Airport Express, East Rail Line First Class, Lo Wu and Lok Ma Chau stations) within 72 hours after swiping it for the first time at the entrance gate. Outside 72 hours, you can top up the card to use as a normal Octopus and the balance can be refunded. One thing to clear is that the Airport Express rides will be valid for 14 days, so you can still use it to take the express train on 4th day.