RH9 seems to have slightly nicer graphical interface and the GUI technology is supposed to be better, but I could not tell much different.
The real thing to watch for is what kernal the OS is using. That is where you can really get confused between 2 versions of RedHat.
From what I can tell, the difference between 8 and 9 is window dressing at the GUI level.
But, indirectly, I did have a problem because I was using both version at the same time. I use 8 at home and 9 at work and only ran into one problem. I use postgres on both. Postgres 7.2 comes with 8 and Postgres 7.3 comes with 9. You would not think it would matter, but when I tried to use yet another program in Windows that connects to the database on RH, the software would not work on Postgres 7.2. This is some of the weird stuff you might run into. In others words, you probably would not notice any differences between 8 and 9 until you really started to use the systems side by side (like one at home and one at work) and tried to do the same things to the boxes (like tried to use the same software on them). Even then, a difference is probably rare, and if you are only running one box, chances are you will install the software for that version only. You wont be a noob like me and try to use one boxes software on the other box.