Well let me go against the grain here. If it's your only ski , I say 650sx. Why , because in my opinion it's more "fun" because it's a true stand-up although many will point out not a very successful one. But if you do the now standard X2 mods , redesign hull , steering , 750 swap your gonna have one bad mofo.
Stock vs Stock a SX is lighter and faster than an X2. Sx's porpoise but X2's plow. Both need correction. You probaly can ride an X2 faster but it's wider and has fixed steering , aka easier. X2's have a strong following , heck I even have one , swapped the engine out of my 650sx into it. I needed a new hull. So pick your poison , you can't go wrong.