If you are doing a clean install, the only difference is that the upgrade
version will ask for a previous version of windows.
Just insert your windows 98/ME disc into the CD drive when asked for proof
of a previous version during installation of XP.
Both CDs have exactly the same content... the ONLY difference is that the
Upgrade edition needs to confirm you do have a previous qualifying version.
The previous version does NOT have to be installed, but you must have the
CD.
Full product is always better.
Upgrades try to keep your old applications in a
functionaing fashion. Unfortunately, this convenience
often creates more problems that it solves.