the sounds are nearly the same. I think the PA800 sounds a bit better.
The Pa500 have no After Touch,
no EC5-connector, no mp3-recorder....
only SD-Card , no USB-Port for sticks.
The PA800 is a full Sampler, the PA500 not.
The Pa800 with mp3-board can Play mp3s on the double-player.
The Pa500 only midis.
PA800 have a coulor display.
Best regards from germany