There are WAY more Yamaha styles available, but I haven't had any trouble finding all the Korg styles I need or want.
My top ten reasons for choosing PA900 over PSR S950 are:
1. The styles sound more life-like, "live".
2. The vocal harmonizer is much better.
3. Songbook
4. Ease of editing and saving, even while style is running.
5. Ability to load style into factory locations, even replace factory styles.
6. Ability to easily load and use third-party sounds.
7. Assignable knob and buttons.
8. Nice touch screen
9. Key touch and feel (although this certainly depends on user preference)
10. Automatic power-on setup, with a dedicated button to recall it at any time
11. Joystick. Woops that's 11 already.
12. Chord sequencer.
13. You can easily add INTERNAL USB drive for storage if needed. I haven't found a need for it.
14. Styles, fills, and other operations are easily accessed via midi foot controller. Only one that I know works for Yamaha is the huge, costly MFC10.
15. Korg periodically issues free upgrades with new styles and features!
In all fairness, there are things I prefer on the Yamaha, such as some of the lead voices, particularly electric guitars. I have found everything I need for the Korg, but it took a little searching, editing, etc.
950 has two assignable footswitches. The PA900 has one assignable and one dedicated to sustain. I use a midi foot controller with the 900.
Another point in favor of the 950 is its ability to play styles directly from USB. Styles must be loaded into the Korgs before auditioning them.
I haven't needed much customer support from Korg, but Yamaha responds quicker and is more helpful.
The user community is vastly larger with Yamaha, such as PSR Tutorial site and Jososoft.com
Hope some of this is helpful. Really the only way to know which best suits you is to spend a lot of time with both. That's what I did.
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DonM