In reply to the cassin ridge:
Choosing a softshell depends on if you are a warmer or colder person. When I climb or ski in the winter I run real hot and overheat easily. The Gamma MX is a great choice if it's cold conditions but if it's warm weather the MX is way too hot for me and doesn't breath enough. I haven't used the Ascensionist before.
If you like the Arc'teryx brand and fit check out the Gamma LT or Epsilon. Both have more breathable fabrics than the MX, are cheaper in price, and far more breathable. If you run hot and want something more breathable these may be a better option than either of your original choices.
Finding the right mix of breathability and weather resistance is tough however most companies use excellent DWR's (durable water repellent)that shed rain and snow extremely well.