Hi Roland, welcome to the club! I have only used down bags, started experimenting with bivys about a dozen years ago, and in the process became a devout adherent. This was before I discovered the ultralight movement. Because the weather in the PNW is similar to yours in Alaska, what I learned through trial and error will be applicable.
* Breathable is better than waterproofness in most shelter situations for an ultralight bivy.
* WP is the gold standard in more exposed situations for a heavier storm bivy.
* For cold weather, weather approaching the dew point, or prolonged wet weather, use a vapor barrier liner! This can not be overemphasized for down bag use, as it will slowly start to wet out from insensate persperation; a VP liner keeps this from condensing inside the bag.
* And don't breath inside the bag! If the air is cold enough to require preheating, adjust your balaclava so that it pre-warms the air for you.
For more information, get the Bivy Sack Review from the print version of Backpacking Light magazine, vol 7. In it 14 bivys are evaluated for storm protection, insect comfort, condensation resistence, packability, livability, durability, weight, and cost. At different ends of the scale, the Equinox ultralight and the Oware Epic came out tops.