Yamaha usually follow a 3 year replacement cycle (The T4 was one of the few exceptions to this rule).

If you want OOTB performance go for Yamaha, if you want to make yourself more individual (Editing setups etc.) then go for the Korg.

As has been said the PA900 is just a smaller version of the PA3x. (Personally I would try and find a used PA2x as it was quite a step up from the PA1x, whereas the PA3x just doesn’t do it for me)

Hope this helps

Bill