If you're looking at the SX:7's, the GS:9's are a step up. If you're an aggresive skier, and ski anything PA can throw at you, definatly go with the GS:9's, or the SX:9's. The 7's are more of an intermediate ski, and the SX or GS 9's will give you a bit more performance.
Atomics are great skis, and you won't be dissapointed. Pick up a pair of 170cm GS:9's from last year, and you'll fall in love. The bindings that come on atomics (and you can only use atomic bindings, the GS:9's usually have the Race412, you an slide the whole binding assembly on the fly forward and backward a little over an inch to distrube your weight differently, and it makes the ski ski totally different, its amazing). Good luck.