Airport Express or Ordinary Train

Hi! For this trip, my wife & myself plan to take the Airport Express. I understand that there is a free shuttle that will connect us to the area of our hotel from the designated station (Kowloon) to the Prince Edward area.
1. Does it also follow for our return trip? Should we purchase a Return Airport Express? But we will be in HK for 6 days. Or do we buy it on the actual day of return, and where?
2. Alternatively, we can take the ordinary train to Prince Edward. Will it be cheaper and as comfortable?