Cash only, last time I was there. Usually what I do is convert USD to HKG when I get off the plane, then go to the Octopus Card information desk (not the automated kiosk) and get a card from them. They usually speak good English. Then on the way out you can go to the same desk and they will refund the remaining balance on the card.

The card will also come in handy if you want to buy a Coke or some McDonald's. But you said you're not sightseeing so if you're just going from the airport to the hotel you might could just get some cash.