I have not used UA or Sitka but have tried numerous variations of synthetics through the years - polypropylene and polyester types. However, since Smartwool came out with merino light-weight and medium weight underwear I quit searching. I wear it every day all winter. It washes and dries easily, doesn't hold odor, wicks moisture well and seems to wear forever. When the temperatures drop to single digits I use the Ulfrotte 200 zip neck over the Smart wool. It is unbelievably warm for the weight and bulk (lack of).