I do a lot of my hiking in central Florida where I live. Summer here is very hot and humid. Temperatures are hovering from mid 90's to mid 100's (F) this summer and with the high humidity, it feels much hotter than that. I've tried just about every fabric you can think of while hiking in this heat, and haven't noticed much of a difference. When it gets that hot, it's really hard for any fabric, natural or synthetic, to perform well. Nowadays, I will just hike with no shirt and some light shorts. Otherwise, the shirt I'm wearing will be stained white from the salt in my sweat after it dries. I'll usually have a long sleeve synthetic with me to wear after I set up camp to help with the bugs.
Now if you live somewhere where there is a chance temperatures could suddenly drop, I would go with synthetic. But in Florida, I don't have to worry about that.