I have both, and they are both excellent; however, the Kuiu is unquestionably better quality. Warmth? There are many products that will keep you equally warm or warmer, but will they serve you as well in what you will encounter in typical elk country? For whitetail deer treestand hunting, Sitka has a wider range of options that Kuiu has not tried to address. But for active mountain hunting where light weight and the highest quality of materials and construction are paramount, Kuiu is the way to go. Jason Hairston has designed and produced some amazing products that no one else can offer, and his prices are competitive with the other less impressive products from other manufacturers because of the direct marketing model that Jason implemented to keep Kuiu affordable to his customers.
In summary, you can't go wrong with either, but for active elk hunting in rugged terrain, Kuiu is easily the better choice. Of course, that assumes that the Kuiu products are still available in your size and pattern/color. For warmth, be sure to buy a Spindrift jacket to go with the other clothing (including an outer shell to go over and protect the Spindrift -- the Guide jacket and/or the Chugach jacket), and get both weights of Merino wool base layers.
Allan