Every weekend save one this winter season, I carried a windshirt into the field. The weekend I left it at home, the high temp was neg 5 and I could not fit enough insulation underneath the shirt to warrant bringing it.
My windshirt is a Marmot Ion circa spring '07. I bought it on a clearance rack at REI for $15. I consider it a steal. It is a size small; normally I would wear a medium or a large, depending on the brand and cut. The cut on the Ion is narrow. The elastic wrist bands securely stick to my wrists and hold the sleaves in place. The jacket goes slightly below my waist. It has a full zip front. The hood is the best hood I have ever seen on a jacket, even better than my Marmot Precip jacket. The hood holds tight to my head and keeps wind out of my ears, even when I'm facing a headwind.
I have yet to use it though, in field temperatures above 40 degrees. Each weekend has been cold-weather camping. I often wear a long sleeve base layer underneath the shirt. This helps retain heat. Otherwise, the shirt is pushed against my skin by the wind and I lose heat through conduction.
When the temps rise though, I'm anticipating that I will need one of two things to happen to need the windshirt. It would either need to be extremely windy to the point where I would get cold, or so cold that even a minor wind would push temps into uncomfortable issues. I don't anticipate using it
On the SHT in May, I'm anticipating a temperature range between 30 and 70+ degrees. it creates minor difficulties on how to plan my clothes selection, specifically my warmth layer(s) and wicking shirt(s).
I don't want to carry a warmth layer that it too bulky for my needs. In past years, I have carried a wool shirt pull over. I going to switch this year to either a thin fleece jacket or a thicker fleece vest. For wicking layers, I need to decide if I need two shirts - one short sleeve, one long.
I'm probably going to go with the thinner jacket because it has sleeves. Although the vest is warm (and heavier at 200 wgt fleece), it neglects the arms. I have chilled my forearms in the past. For the same reason, I'm most likely going to go with two wicking shirts. When it is colder, the extra warmth of the sleeves is invaluble, plus, it gives me a dry layer to change into at night.
Similarly, I'm going to take two wicking layers on the bottom - spandex shorts and a poly-pro wicking layer. It should not be too bulky and I anticipate using all of it throughout the trip.