Sandvich. If I were you and were looking for a pant for backcountry split boarding I would look towards an air permeable softshell pant. They are not completely waterproof, nor are they completely windproof, but they do offer phenomenal breathability and high wind and weather resistance. Check out the Navigation Pant from Mountain Hardwear for an example. If you really want a waterproof pant the Pinnacle Pant would be a great choice. The 20K number refers to the kind of test independent testing facilities do to determine breathability of fabrics not using the GoreTex laminate of which there are hundreds from every brand under the sun. Conduit is Mountain Hardwear's proprietary laminate that exists in many forms that we use in tents, sleeping bags, and apparel. Anyway, the higher the number... the greater the breathability. Often times, you will see two numbers together (20,000/20,000) or something... and this refers to the waterproofness and breathability rating of a given fabric. You should also know that some of our most breathable fabrics exist in the Conduit family of fabrics and are found in jackets like the Terra Shell from Hardwear. As for the two pants you are comparing, the Pinnacle will be similar in breathability to the Synchro... but, the Synchro will be slightly warmer due to the micro-fleece lining. I would go with the one that fits the best! Hope this info helps!