I play a Yamaha upright -- a large one -- for my lessons. At home I play a 5'8" grand.
For me it is a no-brainer that the grand sounds better. This could be partly the better piano, and partly the acoustics of the room.
In any case, some pieces don't sound all that different when played in the morning on the grand, and for the lesson -- later in the day -- on the upright.
Other pieces . . . the difference is great! The richness of sound is just not there on the upright. But the piece being played will affect how striking this upright/grand contrast is.
Here's my question -- what piece would be a good showcase piece to demonstrate the difference in richness of sound between pianos?
I nominate the Chopin nocturne Opus 37, No. 1. I think it is all the big left-hand chords in the C4 octave that really showcase the richness and power of the grand versus the upright.
I hope that if or when people suggest a demo piece, they really think about the sound, and do not just pick whatever happens to be their favorite piece this week.