Many different opinions.

When I was a guitar player / singer, I had to stand – I lost feeling and movement when I was sitting. My voice was also stronger when standing; more room for breathing.

With keyboard it was always a sitting. If I raised the keyboard even with my elbow (piano position) the stand was too high. If there is music on a stand and keys in front sitting is just fine. You need room for moving around the keyboard and freedom for your feet. (Pedals?)

John C.