what's the difference between the superfly and the superfly xt?

Super-Fly XT is a little more of a back country based shell in comparison to the lighter weight Super-Fly (6 oz. difference approx.).....the XT has added re-enforcements (little less breatheable) throughout the shoulders, elbows and lower back/waist for a larger packs hip belt....the cut of the XT is a little more boxier for extra room to fit a puffy or other insulating piece within the main body, a higher cut collar for more protection in shadier conditions, more room in the forearms and still leaves enough room for working with a harness....they added an extra napolean/map/chest pocket, a cinch cord throughout the mid torso, as well as core vented zips by your obliques which I find are much more efficeint at venting than standard pit zips....also, I would take a size down with the XT as they are a little bigger in overall sizing....Either way, they are both great jackets....It just depends on what you want to use if for....for all around usage (running, cycling, hiking, light packing, etc.) run with the Super-Fly....for heavier wear and tear (backcountry skiing, multi-day packing, exped/alpine usage, etc) go with the XT....hope this helps.