I also recommend taking the train. It is easier and faster and the interesting sights are in town......

In any case get the day return for 100 HK$ - not all of the ticket both have it. You can even buy the ticket at the arrival station.
To save time i recommend: walk to the train station until you see the overhead sign when the next train will be leaving. If you have only 2 or 3 min left just board the train and buy the ticket at the arrival station. Same if there is a long line at the ticket booth at the departure station (they take credit cards).