I think I've listened to too many digital piano demos because it was immediately obvious to me which is which.
Same here. The Casio from a demo and the Nord from owning one.
So of course I'm biased towards the Nord, but there is also some specific quality of (apparently) all Casio piano sounds that I don't like. It's hard to pin down, but there is something unnatural about quite a few notes somewhere in the middle register. Basically when I heard the first note of the first song (I won't tell which version), I thought "oh, there's that Casio sound". Not saying the PX-5s is a bad instrument though, for example the EPs seem quite nice, especially the effects.