size and my opinion

This a polyester Sherpa shirt coat that has s smooth (soft, hide-look) interior that is very nice. The pockets are deep. My only concern is I'm used to wearing only natural fibers (wool, cotton, linen mostly) and I figure the less breathable nature of the fabric might trap heat and make it too warm. But it feels cozy, so I'm conflicted. Also, I got both colors and it is going to be difficult to decide which one to keep. Hmmm. I am a Theory size 10 (46 Italian, 42 France), and the small is my preferred size. If I lived in NY I'd get the medium so I could wear over other pieces.