Aiport Express Travel Pass or Octopus Card for HK trip??

I will be travelling to HK for 6 days 5 nights, should I purchase the Airport Express Travel Pass (2 ways airport express ride plus 3 consecutive days of unlimited travel on MTR) as I am planning to use the airport express (round trip) from/to the airport and travel round in the HK city frequently by MTR. However, by doing so, I will be facing problem in my travelling with MTR for the next 3 days as the travel pass only covers first 3 days ride only. Or else, I just purchase the Airport Express Round Trip Ticket - Kowloon Station instead as I am still having my octopus card with me now. Please advise. Thanking in advance.