i will stay in hong kong for 6 days. so should I buy octopus or express travel pass?

Hello, the AE Travel Pass is suitable for you.
The pass contains one or two AE single journey(s) plus 3 consecutive days of unlimited travel on MTR (except East Rail Line First Class, Lo Wu and Lok Ma Chau stations), Light Rail and MTR Bus (Northwest New Territories). The prices are HK$250 and HK$350 respectively.
After 3 days, you can recharge money on the card. Then it can be used widely like the Standard Octopus Card. You will some discounts when you taking other public transportation, like public bus.
Have a nice journey!