I would get 7 because of the installations issue. Also, I'm sure that 8 will be along someday, with great new features, and upgrading from 7 to 8 will be easier and cheaper than from 6 to 8. For software that you think you will use frequently, I would always try to get the latest version.
There was a great improvement in the user interface for 6. I have the Dear Jane software, which is based on 5, and I find it much more awkward.