I can only add to the recommendations for the effiient Airport Express. It also has a free shuttle bus between Kowloon or HK Stations and a number of hotels. Full details
hereDepending on your length of stay, and intended use of MTR, it may be worthwhle getting 2 x Single Journey Tourist Octopus rather than the return one as you would then get 6 days of MTR travel, 2 one-way journeys on the Airport Express and 40HKD of spending for 440HKD. When you return the cards at the airport service centre you'll get 50HKD back for each card plus whatever is left of the 40HKD spending so you'll end up spending 300-340HKD. Individual one-day MTR passes are 50HKD (no deposit). You can use the Octopus MTR days either before or after the Airport Express trip.
The 20HKD spending on each card can be used in various places such as local shops,
the peak tram (sit on the right going up for excellent views) or for MTR journeys after you have used your included days.
There is some rule about MTR journeys in a certain period before your Airport Express journey being free as well (assuming that you are outside of your included days) but I've never worked out what it is!
I've got a feeling that they only return the deposits and unused spending in cash, irrespective of how you purchased the cards to start with.
HTH.
Cheers,
Brian.