we're having a vacation in hongkong on feb 6 - 9.

Well, according to your situation, it's suggested to buy Airport Express Travel Pass. It contains one or two AE single journey(s) plus 3 consecutive days of unlimited travel on MTR (except AE, East Rail Line First Class, Lo Wu and Lok Ma Chau stations). Then on the fourth day, you can recharge some money and continue to use the card.