I recommend the train. Train station is closer to the custom exit than bus and taxi. You can take the luggage trolley right up to the train and just need to lift the luggage from the trolley into the train.
At Kowloon station there are also luggage trolleys just in front of the train door. Take it to the shuttle bus stop, lift it on the bus. At the hotel the bellboys will help you taking the luggage out of the bus.
Easy, quick, convenient.
Buy your train ticket either at the boot at the departure platform (special price for two people, i believe it is 170 HK, they take credit cards) when you have enough time before train departure (every approx 10 min), if not enough time just board the train and buy ticket at the arrival station (same price there).
On the way back just board the shuttle bus at the hotel, buy the ticket at kowloon station and check in at the train station. Take the train and at the airport just go directly to the gate or lounge.