i think it's pretty common for people who are learning to think cork 9s are easier, and more rotations --> more babes so why not right?
but my suggestion is that you work on your cork 7s, get em good, so good you know where you are in the air all the time and can grab and steeze it or whatever.. That way you'll build up your air awareness skills and become a much better jumper in the long run. Especially right now when you're learning, you don't want to pick up any bad spinning habits like looking down or taking off backseat..