I don't know keybeds on synthesizers you mentioned. My guess would be, MODX6 and PSR-SX have similar (if not equal) keybed, as both were launched at about same time (at similar price range).
When PSR-SX was released, some Youtube reviewers mentioned, that SX series has "improved" keybed (compared to previous PSR-S series). If you ask me, that was a plain lie -which was probably based on first impression. Let me explain..
Before buying I did try SX700 in store and indeed, keybed felt somewhat "better" than on PSR-S775 that I had at home. And so, if someone would ask me, I would say "yes, PSR-SX has a little better keybed".
To shorten the story... after several months (of gentle use), keys become "loud". Especially the loud "rattle" of black ones is extremely disturbing. If someone say that's not the case for his/her PSR-SX, then that means (s)he simply forgot how quiet keybed once was.
The only solution is, to take keyboard apart and apply proper silicone grease under each key. The whole job took less than 2 hours -and I've got new keyboard again!
About "proper" silicone grease... I had some silicone white grease, but I could easy recognize that it was too fluid (compared to what Yamaha used). There are many kinds of greases and when buying on web shop, one just don't know how thick, sticky or fluid the grease is. And so I just bough three different greases, by hoping that one of them will be right.
My theory is, if too fluid, then grease won't dampen keys enough and probably the treatment will be needed in short period of time again (as grease is pushed out by pressing the keys). If grease is too sticky, then it will dampen keys too much (keys could feel "sticky" even).
I hope that is also of some help to others who might read this thread.
Bogdan