The difference is huge. Windows Vista is an entirely different operating system, and Vista Inspirat is just a modification that skins Windows XP, trying to make it look like Vista as much as possible.
If you are planning to upgrade to Vista I would advise you to leave the upgrade alone, and consider Windows 7 instead. Vista has a few compatibility issues and is also very heavy and slow. Even on my 4-core setup and on a friend's 6-core we found that if you use vista too much it gets slower and heavier with time, making boot times reach up to 2 minutes and even more. Windows 7, however eliminates most of the issues Vista had, bringing you the looks of Vista (even more beautiful looks if you ask me) and the speed of Windows XP.
In short: If you're OK with XP and do not plan to do an upgrade, yet still want some more visual styles, keep Vista Inspirat - it's lightweight and I have to admit I have used it too. If, however, you are looking for a OS upgrade, try Windows 7 - it's pretty cheap for an OS of its kind and you will find it more functional in many ways.
All that I needed was the one thing I couldn't find...