Airport Express or Octopus Card in Hong Kong

I am going to be in HK for 5 days and 4 nights. We are going to Disneyland, Ocean Park, Tram, Night Market and plan to use mainly MTR. Is it advisable to buy the Airport Express Card with 2 airport express journey and unlimited rides for 3 days or get separately just the Octopus Card?
Lets say I am going to use the card on the 4th and 5th day and I can just top up the card right? But will my 2nd ride of the Single Airport Express journey be forfeited or is it still available?