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.

This isn't fabric weight, just overall weight- size XL is 720 grams.

ive just been in contact with montane and the xt is still currently generously sized/about one size bigger than their other hardshells, the xl is about a 45 inch chest but all other montane hardshells are 42 inch chest in the large size.

i just got the montane halo jacket and the large is too small for me as i have a 43 inch chest but the superfly xt large is a good fit as its closer to a 44-45 inch chest fit.
i have a large in a marmot jacket and fit north face large so i would say get a large in the superfly xt but if your considering any other montane clothing you probably want the extralarge

i have a 2007 superfly xt, im 6 foot, 43 inch chest, fit north face large perfectly.
sizing is odd with montane, the superfly in xl was too big but the large was exactly right however all other montane underclothing i need an xl in so i think they made the 2007 sperfly xt a generous large for layering, i dont know if they still have the generous one size bigger in their hardshells than the rest of their clothing though.

Size up... I wear large in most everything and the Montane jacket I own fits very trim in XL.