Really comes down to whether you like bikes that turn well but like to headshake, or if you hate headshake but have to muscle them around corners more...
86-88 CR500 have good forks that need to be set up properly, but have fairly snappy motors. The 89 motor was smoothed out (ie: improved), but then they got those shit Showa 41mm USDs that need very careful setting up or they'll try to kill you.
88 KX500 is a good, solid bus.
89 US spec KX5 got the wonderful 46mm RWU KYB forks, a better front brake and a few other detail updates and is even better than the 88.
The Aussie/Euro spec 89 KX5 got KYB's version of shitty 41mm USD forks, and is IMO a step backward from the 88 model.
Personally, I'd buy green every time.