Unless you are very interested in historic details I would say that the shorter tour will do it. I would recommend visiting the Carriage Museum, too. If weather permits take your time and explore the gardens; the maze is more for children.
Most visitors arrive at the main gate, do their tour, and then walk up the hill to the Gloriette, which is on the very top. It is much less strenuous to do it the other way round. Enter the premises via the Maria Theresia Tor (south-east garden entrance), already being at the hill top, walk comfortably to the Gloriette. Climb up to its roof terrace for a wonderful view. After that have a coffe and a cake in the café.
It is rather simple to get to the Maria Theresia Tor (Tor = gate) by public transport. At Meidling Bhf. (Meidling train station) bus 8A departs, ride until stop Stranitzkygasse, then just walk straight over the bridge into the gardens. Meidling Bhf. is a major public transport hub, reachable from all parts of Vienna, most notably metro U6, S-trains (Schnellbahn), tram 62, WLB (Badner Bahn). What would be the best choice will depend from where you will be coming from.