A revolution in clothing has been going for a few years now.  Under Armour and Nike Dri Fit seem to be leading the pack.  The material in this type of clothing is designed to pull the perspiration away from your body, keeping you cool, dry and comfortable.  I first noticed it worn by football players and eventually military personnel and law enforcement joined in.  Almost any type of occupation where you have to wear a protective shell or layers of clothing works wonders with this type of clothing. 

I have worn both and found them equal in performance.  The compress or snuger fitting shirts tend to keep me drier.  The loose shirts are good to be worn as a regular shirt.  I noticed that wearing these shirts in places like Las Vegas, Phoenix etc, that I stay dry because the sweat doesn't stay in my clothes.  The other side benefit to these clothes is that if you travel, they are great to bring along because all you have to do is hand wash, wring it out a couple of times and they are dry really quick.