"Which looks best? or which sounds best?

Indeed ~ that's what matters. I am a strong advocate of standing when singing, because it definitely affects the sound of music resonating from within one's body.

For instrumentalists, I doubt that it matters much ~ although, perhaps, some intruments are played more effectively one way or the other: sitting is de rigeur for piano and drum kit, obviously, and standing might be marginally preferable for blowing through wind instruments.

For aging personnel playing through a long evening, I'm sure it's best to sit. The music will sound OK for a longer time than if the players become too exhausted too soon.