Wow, you have some great choices there. Here’s my 2cents along with pros
Yamaha has the best sounds, The most style choices, But if you want a live sound from the styles it may disappoint you.... If you like a very polished studio sound it’s a great choice. The vocal harmony though lags greatly behind Korg and Ketron.
Korg has great features like the chord sequencer, song book,and the best vocal harmonizes. It has a good selections of styles available but not as many as Yamaha. The sound of the styles is perhaps a little more lively and aggressive then Yamaha . I think of it as a nice “punchy” sound.
Ketron has beautiful very live sound in styles. Only problem is their style choices are very limited. If you know that they have the styles that you need, you should be fine.... The vocal harmonizer is quite good. But maybe it’s not as good as it TC helicon that Korg uses, but way better than Yamaha.
Well I don’t own all the top-of-the-line instruments, I do play all three brands, I still haven’t found the perfect keyboard for me, but I do know that great music can be made on any these keyboards your’re considering. You just need to decide what features are most important to you. Good luck!