I know Vantage is DX10 and tests PhysX (gives Nvidia cards an advantage). Other than that it is just a newer version. And is ok for testing and getting a feel for performance. However, I never swear by it as it is NOT a game nor tells you how well a system will perform in games. It tends to favor multiple cores and Nvidia cards.
I use it to stress test. But find that the best stress test is just hours of gaming in the end. I also use game benchmarks as well.
I believe 06 has a free version and Vantage has a free version but only lets you see yoru score once online. I believe the basic version is like $6 and you can run it as much as you want.