I'm using Right Start with my advanced 8yo, my 6yo, and my 4yo. It's very challenging for me, as it uses the abacus as the backbone of the program, and I was never taught to think of math in those terms.
BUT....it's been amazing for my kids, and my 8yo passed a sample SBA test that was one year ahead of his grade level without even blinking. He's sort of learned to think "with an abacus in his head," and that's how I'm slowly learning to do math, as well.
It's amazing that I've spent all these years thinking I was great at Trig and geometry, etc., and then finding out how much simpler so many other things could have been had I learned to do math with something as seemingly simple as an abacus. (But it's not simple if you haven't grown up learning with one...it took me a while to get the hang of it, and even now it's easier to THINK in the Right Start math way than to USE the abacus).
Good luck, whatever you do, but I'd say if you use Right STart with your 9yo, begin with the Transition Math for Level C. We began with Level D and it's a lot to start with (and we started with the Transition book). Plus, it's a lot of card games (some of which he can play alone), so there is more to it than just book work.