In my opinion the MAIN difference is that EQ6 comes with 4 activations. Once used, you will need to pay for additional installations! So if your hard drive crashes, or you reformat to install a new Windows OS, or you buy a new computer... poof you're going through those 4 activations in a hurry!
EQ7 can only be installed on two computers, but all you have to do to MOVE it is to deactivate on one computer, install in on the new one, and reactivate. VERY fair and reasonable.
It does have some cool new features. It's always worth getting the most current version, since they only come out every few years, a LOT of neat things get added.