I will tell you in great certainty that the answer is both. There is no right or wrong or better, but rather which set-up is best or appropriate for a certain gig. The choice between Mono and Stereo should be subject to the case to case differences of various events. It also relies on preference of the artist, producer and sound engineer; not precisely in that order.
Aside from personal preferences, factors that need to be considered are venue size and nature or type of playback or amplified material and speaker placement set-up. For small venues where speaker distances are not that far, stereo and mono applications might even have very slight difference at all. But if you have a set-up where the speakers are so far apart and the playback material has all of its hi-hats panned hard left, the ones on the right would have missed those notes or sound which are essential for the whole listening experience. This applies to all sound source where there is panning involved in the mix. At some instances, stereo would be best if you are going for sound placement effect as in surround speaker system in the movies where you know exactly from where a sound is coming from.
Some productions who are well aware of their sound outputs shift from mono to stereo whenever necessary meaning they have the best of both worlds. As a conclusion, if you are doing a fast set-up and not well briefed on the sound source outputs, go for mono. Best is to have a system rigged for stereo but set in mono with the option to switch to stereo when needed. Also bear in mind that by shifting from mono to stereo, certain elements will be affected as well such as your EQs, compressors, gates and effects so if you think it will be difficult for you to do real time fast calibrations, just donˊt do it. A solution to this is by using digital mixers where you can shift from one scene to another such as mono to stereo or live band to choir with a push of a button and all the audio processing elements adjusted at the same time. Of course, it would have to be set up ahead of time before the event starts.
Summary tip of the day: Know your eventˊs audio requirements and source before you decide for a mono or stereo set-up.