I have had them all. There are no wrong choices.
If you sing, the Korg PA900 has an excellent vocal processor and harmonizer. The harmonizer in the S970 can be used, but pales in comparison to the Korg.
While there is a vast assortment of Yamaha styles floating around the internet, many of them are duplicates; some good, some bad. Korg periodically gives us free factory styles.
I liked the S970, I liked the PA900 more; I love the PA4X, but that's another level.