Okay, here's why I recommend light synthetics over cotton:

In Baghdad wearing 40 lbs of black body armor over my clothes, cotton didn't keep me anywhere near as comfortable as the synthetic materials. The majority of the expats I worked with there switched to synthetic clothing for much the same reason.

Cotton is hydrophillic, meaning it has an affinity for or attraction to water, meaning it gets wet and stays wet. If it is 90 degrees out and you are standing in a good breeze, cotton will help keep you cooler. If it is 120 degrees and you are sitting in a bunker without air conditioning or a breeze, the cotton will eventually feel like wet sandpaper against your skin. If you can't freely move around to distribute the moisture or get enough surface exposure to promote suitable evaporation, the wet cotton will act as an irritant, and you will get a rash.

Synthetics are hydrophobic, meaning the fabric does not like to be wet. When it does get wet, it tends to hold the moisture at the surface of the fibers, rather than draw it in like a sponge. In clothing, this effect is known better as wicking. Synthetics tend to wick moisture away from the body. It will help keep you cooler on a hot day in a breeze, and it will help keep your skin drier when you are stuck in less than ideal evaporative conditions by drawing moisture away from the surface of your skin more. The difference is that one person wearing synthetic clothing will appear under extreme conditions to have sprung a leak as the excess perspiration sloughs off his clothing and falls to the ground, while the guy next to him wearing cotton will look like he's just come out of a sauna and his clothes are soaked from top to bottom.

One big drawback to synthetics is their vulnerability to ignition sources. One little stray spark will make a hole in your synthetic shirt, where it may not even leave a mark on a cotton one.

I used to think cotton was the best hot weather clothing till I went to Baghdad. Now I seldom wear cotton in hot weather, and when I do, I often regret it. Even silk would be more preferable.