I spend a fair amount of time thinking about windshirts. I've owned quite a few. There is a good reason for all this; unless it's raining steadily or warm, when outside I'm always wearing one.
The two which have gotten the most play in the last twelve months are the Rab Cirrus and Zephyr. It is not a coincidence. Aside from fabric and a few minor details, they're identical. Same fit (which suits me perfectly, and is the biggest factor in why I own them), same full zip, same pockets, same (excellent and simple) hood. The Cirrus has a half hem drawcord and elastic cuffs, and is made from Pertex Quantum. The Zephyr has a plain hem, lycra bound cuffs, and is made from stretch fabric, with thicker, tougher nylon weave on the tops of the arms and shoulders. The Zephyr weighs about twice as much as the Cirrus.
I've used the Zephyr a lot more often. It breathes better and is thus more comfortable across a wider temperature range, especially at temps above 40F. Over the summer I found a baselayer tshirt and the Zephyr often got the job done. During real cold (<15F) the enhanced breathability prevents moisture from condensing within my clothing system. The Zephyr is also more abrasion resistant than the Cirrus.
The shortcoming of the Zephyr is in moisture retention. It sucks up a lot more (either sweat or precip) than the Cirrus, and takes significantly longer to dry. I find myself reaching for the Cirrus in that window of temps between 45F and 15F, when for a variety of reasons its just harder to stay dry. The Cirrus is also the obvious choice when its going to be exceptionally windy.
It may seem excessive to have two windshirts, especially given that they are not cheap. Given how much they get worn, I think the expense is well justified. Additionally, my very expensive Goretex anorak is in almost new shape after 3+ years of use, because the windshirts take most of the abuse.
Which would I keep if I could only have one? Fortunately I don't have to make that choice.
If I'm going on a more aerobic endeavor in dryish, warmish weather I'll take the Zephyr.
If I expect high winds I'll take the Cirrus.
If I'm expecting lots of drizzle I'll take the Cirrus.
If I'll be doing lots of bushwacking and/or scrambling I'll take the Zephyr.
If I'm taking a windshirt as a rain coat proxy, in a dry environment like the Utah desert, I'll take the Cirrus.
If I were going on a long hike with unpredictable weather, I'd take the Cirrus, unless if were summer, when I'd probably take the Zephyr.
The ideal would of course be a fabric which blends the best of both. There is; Pertex Equilibrium, unfortunately I've found the readily available variants to not be tough enough for my needs.
