The long tracks are definetly better for the moutains. It will give you more flotation in traction in deep snow with a larger foot print if you will. The short tracks are great for groom trail riding. It might leave you with much to be desired for if you plan on going up the mountain.
I would put no less than a 144" track on it for riding in the mtns or in deep powder. I dont think you will have as much fun on a short track in the mountains, you will have a sore back though if you keep it short.
BTW...I have heard that srx's make excellent mountain sleds. Have fun, and let us know what you do.