I have both the Ti Goat Ptarmigan and Ti Goat Bug bivys. I use both in either standalone, aka Cowboy camping, or in combination with my Zpacks 8.5 x 8.5 flat tarp. I also use the Ptarmigan for snow camping, in combination with my Rab Summit Mountain tent, to add a few degrees of warmth to my sleep system and to deflect moisture brought into the tent thru the front door.
Although I have never had any serious moisture issues with the Ptarmigan, I prefer the Bug bivy as it provides more ventilation which allows me to better regulate my body heat. If the weather forecast is sketchy then I'll take the Ptarmigan. Otherwise, I prefer the Bug bivy. I should note that the vast majority of my trips are in the West (CA, ID, MT), which tends to have predictable, dry weather.
I've used several other bivys, including several Goretex models, and I do not find the Ptarmigan to offer any greater or less breath-ability. I purchased the Ti Goat's bivys several years ago and I have been extremely satisfied.