OSX
I have moved to OSX.3 on all of my machines (see sig) and all of them behave very well, although the iMac is a little slow.
I loved OS 9 until I met OS X, the first day I switched System Folders and booted into OS X I loved it, but it wasn't quite ready for me until Jaguar. Once 10.2 came out I've stopped using OS 9 altogether.
Personally, OS 9 did seem a little faster before OS 10.3 but now I really can't see a difference. Also, OS 9 was good, but OS X is more stable and more user friendly than OS 9, the dock is very useful, the new iApps are great, the command line, Preview, Safari, iTunes and iTMS, Konfabulator, etc. I'm more interested in my computer now than I was with OS 9, I don't feel like I'm going end up with an extensions conflict or get a Flash module that doesn't work properly in Netscape.
My iMac running OS 8 - 9 crashed once a week, sometimes more, my PB has had a kernal panic once or twice since I started using OS 10.1.1.