Hongkong Octopus card

We are a group of 6 persons arriving Hongkong on Aug. 18, 2011. Which is the best option:

1. To purchase Group single journey tickets for 6 persons and regular Octopus card HK 150 for 4persons and 2 elder octopus card at the same time? ( the group single journey will be used at the Airport express going to Kowloon) or

2. Purchase 6 Airport express travel pass (Don't know if we could still use this on Aug. 22 since my computation of 72 hrs for unlimited use of MTR ends on 12noon of Aug. 21, 2011, if I'm correct)

Can we use the Airport Express Travel Pass for payment in convenience store and up to what amount, if ever?

Thank you very much.

God Bless


Daisy