How do I choose between the cotton and the polyester?
This is a tough one to answer. As far as effectiveness goes, they both offer the same basic degree of protection from bites. The density of the weave is approximately the same, so the sun protection factor and durability are pretty much the same.
The main difference between the two fabrics is their moisture absorbency. The cotton is more absorbent, so on those hot sweaty days you may find the cotton fabric absorbs your sweat more, potentially making your skin feel a little drier. The down side of this is that the shirt is then damper. The polyester is less absorbent and consequently quicker drying, so if you’re caught in a downpour while out paddling, you are going to dry off faster in the polyester than you would in the cotton.
Also, the polyester does not show stains as much as the cotton, and there is no shrinkage in the polyester, compared to a shrinkage factor of about 3% in the body and sleeve length for the cotton. The polyester washes up well, whereas the cotton will look a little wrinkly.
The differences are minimal, and it really comes down to what you like the feel of on your skin. The polyester does not have a plasticky feel to it – it has quite a soft hand.