Synthetic vs. Natural, which do you prefer and why?

What do you guys use? Synthetic layers like polypropylene and "wicking" under armor type materials, or Natural layers like wool and cotton?

I use both depending on the condition but I prefer cotton for layering because it still breathes better than synthetics in my opinion. I use thicker cotton and more layers when it gets cold. Sometimes wool, but wool is almost too warm for me. I do have some polypropylene stuff but I sweat too much in it, doesn't breathe well.

I use all under armor and it works for me, however I live in the south. I am looking into wool this year.