Aside from the stiffer, more solid feel of the 9kpro - they keys depress further down than on the psr9k. The feel of the two is definatly different, and the only way I can imagine someone NOT knowing the difference is if they play very, very lightly, and never FEEL the actual resistance factor and the key stroke. I really liked the action on the pro, but didn't like the psr at all.
Remember - I am an old "Rhodes" man - I like to dig in to the keys for expression.

I still think the 9kpro is a GREAT choice for a controller in a larger setup or a stuidio piece. I just have too many other needs for a "live" rig.