In Ears: Mono or Stereo?

Friends,

I'm about to upgrade my mixer and looking at the Aux needs as a major factor for which direction to go.

Everyone I play with does MONO for their in-ear monitors. I've heard that some folks like stereo in-ears.

Can anyone give me any insight what % of pros prefer Stereo vs Mono in-ears? Also, can you tell me why someone would want a Stereo mix vs Mono, what is the sound difference?

Thanks for your insights.

Dan