I have noticed this. When played through a PA, some of the synth, bass and drum sounds on the Korg Pa500 are much fuller than the Yamaha PSR S910. The guitars and saxes(not all saxes sound good with the PA) are much better on the Yamaha. As a slight deviation from the topic, I still find the Roland products whether its a BOTL or TOTL, they sound great out of a PA. Drum and bass are great on nearly every Roland arranger out there.

To get back to the topic, price wise, it would be fair to compare pa600 and S750. Third party styles(thousands of them available for Yamaha), ease of use(what you are used to is a lot easier to migrate to), dealer support, style genres you play(Some of the styles are over done on Korg, Ballad styles are great and non interfering on Yamaha) etc. are all going to influence your choice. And to my amateur ears, the tonal quality in the demos for Pa600 is slightly better than the older Pa800. And the S750, I haven`t heard demos yet but the way Yamaha trickles the technologies every release, presume it will be the S910 sound engine which is also good to my ears

Good luck in your choice. Whatever you use its the right one for you till a new model comes up