I did notice the Drum kits were improved on the S750/S950. Although I think the Korg PA600 Drums sounded livelier. At least from what I've heard on the internet at least. Since I haven't played a PA600 in person I'll have to reserve judgment until I do play one.
Two advantages of the PSR-S950 are a Mic input and a vocal harmonizer. The PSR-S750 lacks both which makes it the direct competitor to the PA600. The downside for the PSR-S950 is the lackluster key action, although I did notice the keys were a little bit better than the S750 key action.
I'm debating whether to bite the bullet and just get the PSR-S950. I really would like a vocalizer and the PA600 doesn't include one. External units are an option but an on-board unit is less gear to haul around. You do pay a high price for those two added features on the S950 i.e. $800 extra vs. the PA600. Although the S950 also includes the "audio" style functionality, minus the looping feature of course. I heard a new demo on YouTube and the guy really made the S950 shine. I especially liked the string, guitar and woodwind sounds in the demo, although the PA600 string and guitar sounds are very good also. I think the Yammie has the edge on the guitar and woodwind sounds though. The Organs and Brass on the S950 are also very good in my opinion. As are also the other categories of sounds. Decisions decisions.
If I splurge on an S950 I'll have to pray I can readjust to the sub-par key action. I'll have to make sure I do hand and finger exercises (on something other than the S950 obviously
to keep my muscle tone up to snuff.
Let's face it. The PSR-S950 is the closest thing to a Tyros4 at half the cost. And the S950 has advanced features that the Tyros4 doesn't have. Decisions decisions.
All the best, Mike