If you use dynamic population, the render times cannot be compared. With dynamic population, Vue7 populates while it renders, Vue6 populates before rendering. Furthermore, if you are using the new spectral 2 clouds, again you cannot compare with Vue6 because the new clouds are more complex than those of Vue6. I find Vue 7 renders faster than Vue 6, in my average scenes.

For some hints on render, see this thread:

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3357632

There's no technical problem in having Vue 6 and 7 installed on the same computer, or even running them at the same time, provided you have memory for both (I'm not sure about the legal aspects, though... if you bought Vue7 as an upgrade to Vue6, legally I think you can not run Vue6 anymore).