The pros and cons of each seem to be equal. Buying at the gate, you may have a slight wait, but you can pick your tram time based on when you arrive. However, during the tourist season some may be full. Buying online you can guarantee a spot, but may get slowed down by the wait through security, which is usually only longer during the tourist season.
I found online to be easier
Buy online because they usually sell out several days ahead of time.
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