Price Difference

• The full version of Windows costs about $100 more than the upgrade version. This is because instead of installing the entire version of the new operating system you're only upgrading the old version.
  
  

Technical Requirements

• Some versions of Windows cannot be upgraded. For example, Windows XP can be upgraded to Windows Vista but not to Windows 7. Even if the version of Windows you're using can be upgraded, the computer needs to meet the minimum requirements of the new operating system or it can't be upgraded.
  
  

Full Version Preferable

• Upgrading Windows isn't recommended. Windows is such a complicated operating system and everyone's configurations and program installations are so different that it is very difficult for Microsoft to cover all possible contingencies. The upgrade version may work just fine but if not it may run slower than the full version or have a lot of bugs. The only way to be sure your new version of Windows runs at optimal performance is to use the full version and totally overwrite the old.