I think it is very difficult, if not impossible to compare keyboards based on a live performance. I'd rank the importance of the various factors in roughly this order:
1) The artistry of the performer on that occassion.
2) The sound system and speaker setup.
3) The artistry of any backup musicians.
4) Acoustics of the room.
5) Roughly tied with 4) -- The quality of sounds on the keyboard used.

10 or 15 years ago, there was so much variation in different keyboards, number 5) might have been tied with 1) or 2). But today I couldn't conceive of comparing two keyboards unless I could demo them side by side through identical speakers. And that's just for a preliminary guess. To be really fair, I'd want them side by side at home for a month or two.

R.