A couple of people have asked me about the XX vs. XTR 10 speed groups. I used XX all last year and it worked pretty flawlessly. I remember two instances of front chain droppings when shifting from the large to small ring. One would be my fault, the other I blame on SRAM. The rest of the time it was perfect. My other bike was setup up with a gripshift/XTR 2x9 combo last year and in comparison it is nice to just leave the front in the big ring and be able to have such a large range on the back. The small ring is too small for anything other than extended climbs.
I havent ridden the new XTR very much yet but I like it so far. It actually is smoother shifting on the rear and I like the way the XTR shifters work a little better. Its nice to be able to shift multiple gears in either direction, I always favored gripshift because of this. The XX crank is carbon and looks cool but they both seem to do the job just fine. Front shifting is about equal I would say, maybe the XX is faster. I dont use it all that much so as long as it stays in gear im happy.
The XTR brakes feel much better than the XX. I had a lot of problems with the XX brakes last year and had better luck with the lower level model from Avid. The XX required bleeding a number of time last season so im hoping the XTR will require less work and headaches. One complaint with the XTR is the lack of matchmaker clamps. There is way to much going on with two brakes, two shifter, and two lockouts all hooked to the handlebars. The XX looks much cleaner. I havent looked into it much but I would like to get XTR rotors for my Stans wheels but I doubt they make a five bolt rotor.
I dont think you can go wrong with either setup. I would probably pick XTR for a new groupset unless I they made a Gripshift version of XX. I dont know the weights of the two, but I would guess the XX is a little lighter.
If I had one or the other, I definitely dont think it is worth changing to the other. They both require you to pedal to make the bike go.
Additionally, I dont have any good reasons for picking 26" wheels over 29" wheels other than they are lighter, quicker, and ive already got a garage full of them. It will be interesting to see if Willow rides big wheels now that she has both available to her. She has said that the 29er had a lot to do with her recent success, but that might have just been marketing.