Joe, I bought a windshirt to do a few things:
1. Block most of the wind to reduce convective heat loss.
2. Block a light rain for a short period, including a hood for my head. (enough time for me to get under cover, get out my rain shell, or set up a tent)
3. Have some air permeability to allow moisture vapor (a gas) to escape readily.
Features I want:
2. Full zip for venting and wearing backwards (on belay, pack on, etc..)
3. Be as light as possible; around 100grams.
Waterproof "breathable" materials such as goretex are not air permeable, so they only meet 1 and 2. eVENT meets all three, but is not light. Most people choose something in single layer nylon. I tried a Montbell 'Windblaster', and a 2006 Marmot 'Ion' before settling on the Patagonia 'Houdini'. The latter one meets my requirements while weighing about 105 grams. I highly recommend it.
Sorry, I do not have experience with the two choices you are considering; but certainly someone here does.
Before you buy a windshirt, try a simple breath test; place your mouth up against the fabric and try to blow through it. This simple test result correlates exactly with how well the fabric transports moisture, in my experience. Goretex the worst, eVENT and EPIC better, single layer nylon the best.
With any single layer nylon, you will need to keep the Durable Water Resistant (DWR) coating in good condition for water repellency. When water starts to soak through unusually quickly; wash the jacket with ivory soap (or similar non-detergent soap), drip dry, spray on a DWR, and iron it in (or toss in dryer).