Short answer… Fresh Install > Upgrading

Long answer… it depends on how the upgrade went and what you are looking for. I recently did an upgrade from Leopard to Snow Leopard and noticed very little problems. the most noticeable problem was the long bootup time… and I know that Snow Leopard had specific optimizations to decrease boot time. Furthermore, I had some apps that were incompatible with Snow Leopard… so I finally decided to give the system a good wipe and install from scratch. and since then… all the problems have been solved… especially after installing the 10.6.2 update. Just always remember to backup your data and apps before wiping things out. Time Machine is very useful in this respect.

On the Windows side, I successfully upgrade from Windows Vista to Windows 7 without a glitch. The two systems are extremely similar under the hood, and I did not notice any problem after upgrading. I even did a clean install of Windows 7 and noticed no improvements. To be fair, I didn’t have anything installed in Vista anyway… just Office and Adobe CS.