I bought Studio V9 several months ago and whilst this was my first purchase of any editing suite, I am pleased with it.
I'm not really able to answer your question directly (that is compare V9 with V7) but when I was "researching" my purchase, V9 appears to be favoured over V7 for several reasons; not least in that it does'nt crash or stall.
I bought V9 because it now supports widescreen mformat - is this important to you?
V9 is apparently designed for AVI file-type but only type II, so this has caused me an issue in that I can't import video from Microsofts Movie Maker(its much better in making titles)without first converting it to type I - which to be honest, is no big thing. As far as I know, there is no compatability problems between V7 and V9.
Reading back what I've just written, is probaly no help whatsoever to you so go to this site where they have a whole forum section on Studio software: click here