The times I have played the piano, I like the hammer action, as it seems more suited for the instrument, after all it is a percussion instrument. And I think it is more expressive....
For everything else I like a medium weighted synth action...like on PRO synths or Genos, PA4, etc.

I will say the SX-900 keybed is very nice indeed...way better to me than the other PSR's were before it.