As the V8 is not available, it will be difficult to give some feedback.
Also, when you say difference between V6 and 7, you have to know that there is much moredifferences between v6.0 and V6.1 than between 6.1 and 7.
The higher the version is, longer the support will be. I don't know how many time you'll have support for 6.11 (I suppose that the 6.0 is no more supported).
With 6.11 and 7, the security has improved, new functionnalities/connectors etc., some APIs are deprecated (for example you can't use anymore E4X into the deliveries content), UI changes (and the UI changed twice : with 6.11 and again with 7), it will be difficult to be exclusive here.
There is also some improvements with the build releases (and not only major versions).