Gateway Marco Polo to HK Airport via A. Express

We're taking the A. Express going to Gateway Marco Polo. how bout going back to the airport? Can we take the free Airport Express Shuttle from our hotel going to Kowloon Stn.of the Airport Express? Should we buy a Rountrip A.E ticket or can we just buy Single Journey ticket at the Kowloon Station (going back to the Airport)? Thanks!