X9 is decent, I've never ridden X0 but people say (like most SRAM stuff) the performance is similar and the only difference is the weight.

Since you are changing the rear der now is the time to think about if you want to go to a double crank, if you ever do want to change.

Of course if you put X9 on the problem is you will then want to upgrade your rockhopper frame

ed, there is an X9 badged crank now.