Hundreds of different sounds have been created to solve various user needs. They are free to download. Another advantage of having a Kurz keyboard.
Ah yes, the options aren't merely "hope the sound is in there or figure out how to make it yourself" -- there are also user communities sharing patches. Kurzweil has Ksetlist, MODX has Soundmondo, and someone's done the sound for that one too...
https://soundmondo.yamahasynth.com/voices/49073
So then getting back to this...
Kurzweil PC4-7: $1,800 (a little above my price range)
Yamaha MOXF6: $1,000
Yamaha MODX6: $1,400
I don't mind spending the extra $400 for the MODX6 if it has better sounds, or more of variety of sounds, than the MOXF6.
MODX does have new horn/string/piano/EP/synth sounds besides the ones that are in the MOXF... the set of sample data in the MODX is larger and more flexible in how it's used, plus they added the FM synth. So there's definitely more variety, and there's a good chance you'll find some things you like better. It also comes with a gigabyte of expansion memory for loading additional sample sets (so for example, you can load the EPs from
https://www.purgatorycreek.com/index.php/montage-modx/ ). Seamless switching (on sounds consisting of no more than 4 Parts) means that a held/decaying sound won't cut off when you switch to another. Sound editing or assembling combinations is easier on the large color touchscreen. And the aforementioned Soundmondo give you easy access to patches from other users. (Dave would probably want me to point out that the Kurzweil has many of these advantages too!)
As for PC4-7 vs MODX overall, from something I posted elsewhere:
PC4 advantages:
* 4 engines: sample-based, FM, virtual analog, clonewheel organ (MODX has the first two)
* assignable additional outputs
* aftertouch
* better (though arguably a bit more complicated) implementation of seamless switching
* more flexible MIDI implementation
* full sequencer
* more polyphony
* more controller support (ribbon, 4 foot switches, though no superknob)
* hard button patch recall (though touchscreen also has advantages)
* full 16 zone/part support (MODX supports 16 internal Parts but only 8 can be assigned to the internal keys, and it supports only 8 external parts)
* double the User sample memory (can actually load a larger version of the Puragtory Creek EPs I mentioned above)
MODX advantages:
* touchscreen (also, screen is bigger)
* built-in USB computer interface
* endless encoders
* their various "motion control" related enhancements, which I admit I've pretty much ignored, it doesn't seem to apply to anything I do, but there is an overview at
https://usa.yamaha.com/products/musi.../features.html - I assume some of these are beyond what the PC4 can do, though I don't really know.
...though not all of those things are necessarily meaningful to you. Which one is sonically ahead specifically for horns, strings, piano, EPs, and synth, I couldn't say for sure. I doubt you'd be disappointed with either.