[BScout]

No difference other than new plugins only come out on V10.
You can run v9 side by side to v10 (though Waves is sneaky and tries to get you to change those v9 to v10 during the install process of v10 plugins -- just reject that.) You can still get the v9 installers from Waves (who acknowledge you can run them side by side.)

There's likely no issue between it and Catalina either -- the plugins are 64 bit already in v9. So it only matters about the version installer. v9 installer is not 64 bit. v10 is 64 bit. So if you need to do an new install once on Catalina, you are going to have problems with the v9 installer (if you upgrade in place you shouldn't because you wouldn't have to run the installer)