Both *are* really good; in a nutshell I find the PA1000 punchier (especially bass and drums). the Yamaha 'cleaner".
Strings is a very subjective category; I prefer the cleaner 'full bowed' orchestral strings on the Yamaha, mind you some people find that screechy... the Korg wins in 80s polysynths IMO, Yamaha in Sax & Woodwinds, strings (imo), brass, but even 9 after years on the market, nothing beats that low end punch of the PA1000.
Ass for expansion packs, it depends *where* you are; for Korg I can only get Turkish& Arabic, and Indian - one of my buddies has PA1000, loves it, but is *very* jealous of the world sample packs available for PSRsx920, especially the new Cpop Chinese pack (the old Chinese pack 2013 was <20mb, the new one (2025) is 450mb so you can imagine how much better it sounds). And there are *so many* free packs to download; Middle Eastern, Spain & Portugal, Italy, Brasil, Dangut, Indonesia, Celtic, etc.
It is definitely best to try them in person.
Oh, there's a very interesting difference nobody seems to mention: the keys: The PA1000's keys don't feel even as good as the old PSRs970/975, very much on par with the feel of the PSRE series.
*HOWEVER* since the introduction of the Yamaha sx series, while the keys initially feel great, the keys become clackier with time... it doesn't bother most people, but the people that it does bother? It *really* bothers them.
What it boils down to, I've found, is if you play on headphones (and you're listening to the keys before you put headphones on) or playing at low speaker volume, you will hear the clack, especially the black keys. I play with the volume up, pretty loud, so I don't. And yes the new sx920/720 share the same keys as the sx700/900.
*AND* the PA1000 has channel after touch; the PSRsx's do not. And the keys never develop a clacking issue (neither did the predecessor Yamaha PSR-S series, but they also lack A.T)
Finally, for me, the deal breaker is whether or not I can chain registrations together for continuously updating the sound scape as I play, like in my youtube videos: This registration sequencing is only on Yamaha's for some reason.
But that's for *me*. I doubt most people ever even use this feature.
*Conversely* the Korg's have seamless sound switching, the Yamaha's don't (which is doubly ironic because you really need that with registrations)... I achieve no pops when switching through some aggrivating techniques; Korg *does* have song book, which lets you instantly recall a registration, I think 16 on a single screen? But you just can't chain them hands free the same way; but if you only need to load a song setup at the start and don't change on the fly, you're good!
Mark