They tend to leapfrog each other. The "Studio" upgrades come out inbetween the "Pro" upgrades. Each upgrade gets some features from the other, and introduces some new features not in the other yet.
In general though, the Studio versions correspond to a number one greater than the Pro versions. So Vegas Movie Studio 9 corresponds to Vegas Pro 8, or 8.5, depending on how you want to look at it.
It's strange, because the studio versions skipped over #5. If they hadn't, and Vegas Movie Studio 6 had been called 5 instead, it would have made a lot more sense. (*shakes head*) go figger.
Given that SCS have released another version of Movie Studio 9 last week (Move Studio HD 9), I wouldn't hold your breath for a v10 release any time soon.