If you are comparing an apples to apples source, in my opinion the quality difference in the sound would be negligible. I currently have an Onkyo TX SR573, and purchased the TX SR607. After a week of toying with it, I was still unable to configure it (IMO) to sound as good as my trusty old 573. My problem could easily be my ears have been listening to an incorrectly configured system for 3 years and anything different from that sounds odd to me. It just sounded very soft.
With the 707, there is no doubt that you are upgrading features/functionality. If you are able to configure it correctly, it should definietly be on par if not better than the 605.
The 707 will be a hit. I plan to purchase it in the coming weeks, mainly to have a receiver with HDMI switching. Hopefully I have more luck with it versus the 607.