Hi fab - Welcome to w7forums
There are no differences other than the upgrade requires you to have a previous OS to attach the upgrade license to. The Full version doesn't require you to own a previous Windows license.
Upgrading XP requires a clean install (this can be done with either upgrade or full version)
Upgrading Vista can be an upgrade or clean install. the choice is yours. (Once again this can be done with the upgrade or full version)
Here is a chart that will help with your question. -
Upgrading your PC to Windows 7