A good, loose fitting, light colored, long sleeve shirt is 100 times better. I have a few shirts that are designed for fly fishing in the tropics, and they are vented to allow airflow.
When I did a desert hike in Utah, a few of my fellow hikers poked fun at the way I was dressed. They were in shorts and T shirts. I had long pants (very thin), a long sleeve shirt, and a big floppy hat.
The heat didn't affect me nearly as much as it affected them (I stayed cooler because the sun wasn't hitting my skin), and I was the only one who didn't return with some degree of sunburn, even though they all applied suntan lotion.