interesting topic.
i also have a korg pa4x and a yamaha genos so i can compare them really well and would like to add my thoughts about it.
My opionion is that it's a matter of personal preference, both keyboards are top of the line but also use a quite different approach.
i agree with all the comments in the previous posts, here are my thoughts regarding some of the comments:
Yamaha Registrations versus Korg songbook:The yamaha registrations are more like snapshots from your current style setup and also include the style settings. The Korg songbook only has one registration (if you compare it to a genos), however, it uses a different approach, in Korg you can change anything in that style without destroying the original style, this goes really far, you can program new sounds for that specific style, You can change elements of the style (like different sound mappings, other bass patterns, etc), you can create chord sequence to play along for that specific song. Custom multipads and much more. It's extremely powerful and the heart of using a korg pa4x and one style or song registration can be changed in a way that it's impossible to recognise the original style (which still will be the original in the style library).
In the genos this is not really possible, if you make changes that way you have to save the style as a new style, also chord sequences are missing, the reprogramming of sounds on a genos is limited to a few parameters, etc.
Yamaha multipads versus Korg Multipads:Here the yamaha has a much better implemenation then the korg. It's so easy on a genos to create your own multipads (audio or midi, even custom arpeggiators) with almost no limits to audiosize. Korg has very limited functionality on this part and the Genos is miles ahead in this area.
Styles:This is a matter of preference. I think if your goal is to play covers that are sounding close to the original arrangements then the yamaha genos is the way to go, there are so many song styles available for the yamaha genos that the list seems endless. The korg pa4x has a much more limited list of styles. Also there are only a few song specific styles available for the korg Pa4x.
Sound wise the styles are mixed differently and also the density of styles are different. The Genos has complete arrangements and sounds more like playing the melody over a cd arrangement. The Korg Pa4x has much sparser styles where you have to do a lot more yourself to get close to playing a cover. The Korg Pa4x is more like playing with a live band.
One thing the Korg has great support for new styles, every 6 till 8 weeks there are free bonus styles available on their website in different genres that you can download for free. This is the case for almost anything on a korg, once you buy a korg then almost anything is free.
Updates:The korg pa4x had 2 major updates in the last few years. These added lot's of new functions (like a new Kaos Effect Processor, a new very high quality piano, enhanced memory to 3 Gigabytes for your expansion packs by introducing sample streaming functionality and more new sounds, styles, etc). Every major update make it seems like having a brand new Keyboard. And all these updates are free of charge.
The Yamaha genos doesn't have this major features in their updates, the updates until now seems to add only minor addons to the feature set. Maybe in the future it will add major new functionalities?
OTS versus keyboard sets:If you create your own sound setups (by combining the three upper and one lower element to a total sound) then the Korg Pa4x has a feature called Keyboard Sets, you can save your own sound sets to a library which you can call up anytime you want, even when playing live you can change infinitely between this keyboard sets.
Yamaha does this through the OTS feature, however, those are limited to the selected style. If you want to do something compareable then you have to create up till 10 registrations for your specific song. The OTS is changed depending on the connected style (unless you use the freeze button), but it is possible, although not as convenient as selecting realtime keyboards sets during play.
Creating your own arpeggiators:Both keyboards can do this, but it is much easier on a yamaha Genos, for instance if you want to connect a new arpeggiator pattern to your left hand, then you can do this within minutes in the multipad midi creator and assign this to a multipad.
Creating your own styles:The korg pa4x can analyse your midi files to create a new style from it. The results are sometimes really good and sometimes really bad (depending on the midi file). There is a feature on the korg pa4x where you can import a style created in your daw, those results are always really good if you set the labels correctly in your midi file.
The yamaha Genos can create styles onboard by recording every element or by merging elements of styles to create a new version (assembly). This is a very easy proces (much easier then on a korg pa4x), so it's a breeze to create great new styles from elements of the thousends of styles available on a yamaha genos.
I think that for the majority of users the Yamaha genos way is the best way to do it, since it's so easy to do.
Style locationsThe korg pa4x has memory slots for every style. Also they can add extra memory slots by using a SET file on a usb stick, but still the memory location space is limited.
The yamaha genos is unlimited, it can load styles in realtime from your usb locations and has a much better system for style management.
User friendly:The korg pa4x has a very difficult to understand interface, it can do a lot (it even has a fully fledged synthesizer including wave sequencing! on board), but since every button has multiple implementations you need to study a lot before you know your way around a korg pa4x.
The yamaha genos is very easy to understand, the user interface is straight forward and i didn't need much time to understand the feature set.
My conclusion:There is no good or bad, it's all a matter of preference. Both keyboard are using a very different approach so they complement eachother very well. My opinion is that if you want to play covers in a convenient easy way with lot's of available song styles then the yamaha genos has no competition. If you are looking for a more complex keyboard with lots of programming features and more ranged on styles that sounds like you are playing with a live band then the korg pa4x is a more obvious choice.
Combining both board will give you everything. I'm glad that i have both keyboards, it's not nessecary of course, but it's a lot of fun