Squeak,
PSR1000 is only $100-200 difference from PSR550.

It is WAY better than 550 in all respects.

-Variations 4
-Registrations (Instant) 8 without switching banks
-Warmer internal speakers
-Better logistics and looks that makes you want to spend more time on it.

In terms of sounds after it's been amplified: well, I think it's going to be quite close although I cannot verify this for sure.