Not an expert here, but I like to run (2) speakers, in stereo. A keyboard has more frequencies than any other instrument, and IMO, is designed to run in stereo.

After MANY PA systems, including MONO, I just get a fuller sound with a speaker on either side of the keyboard. Most of my audiences are in that perfect sweet spot too. If the audience field is wider, I just widen the distance of my speakers.

The gang here will be split on the stereo scenario. Just do what you think sounds the best