I own 2 Yamahas (S910 & Tyros4) and also onwed a PA500 and now a PA600.
Comparing PA600 with 910, I feel like PSR910 has more polished sounds and worse drumkits and feels more like a home keyboard or learning keyboard. I believe S950 is better than S910 but not very much.
Besides S910 has very better acoustic sounds, overall I think i wouldnt be the best choice for a professional musician that would play a wedding reception or party, I think it's more for a casual musician. (of course can be used anyway for these purposes).
On the other hand, Korg is a fully programable machine, its seems a computer, the musician can customize all the keyboard his own taste and needs. Besides it sounds flatter (seems like less ou none dsp), its sounds very very good at an amplifier and I believe a serious musician that has good skills at programming would always prefer the Korg.
I live very much the PA600 and also Tyros4. Both are different and together I have all the things I like in each one.
Between S750 and PA600 I would choose PA600. Between S950 and PA600 I would also choose PA600 becaise it delivers a lot for its price. S950 is overpriced in my opinion. S950 only if you need VH and audio styles to justify pay a lot more. PA600 has better drums and besides it's not a smpler , it's possible to load sets with new sample instruments or drumlkits. Yamaha only paying for Yep expansions.
By the way I like my Tyros4 much more than PA600, but it costs 3 times more. They can't be compared, but I can say PA600 is more programable and customizable while external Yamaha styles are mush easier to try and load.