I ski the X-mountains and love them, possibly the greatest ski ever made and definitely the greatest ski I've ever skied. I'm 5'11" 180lbs and ski the 184s. I ski them everywhere on the mountain but they truly excel in the powder and crud conditions. If you can find a pair buy them and thank me later
I've never skied the super mountains which basically replaced the x-mountains a couple of years ago. The biggest difference seems to be the shape, the supers are much fatter in the tip and the tail but pretty much the same width at the waist.
That's all I have to say about that.