If you are running a triple in the front, you can get away with a mid cage, assuming you will not use the extreme gear combos. If you are running a dual ring, go with an X9 shorty. I just put one on my RFX and love it. Lots of clearance, really quick shifts, and less chainslap.
Larry at Mountain High has them for about the price of an X7 at my LBS.. Plus, the X9's are stronger. The composite cage worried me a bit, but has taken a beating in its first outing.