It's all personal preference. If you look at the Badwater Ultramarathon, you will see some people wearing synthetics and others wearing cotton. More than just the materials, I also pay attention to the weight of the fabric. I don't like the military/tactical type stuff because I find it too heavy for my needs. I like something light and comfortable that won't chafe when covering lots of miles.
I personally wear REI Sahara pants and then some form of thin, long-sleeved, synthetic shirt. I have a 1/4 zip long sleeve jersey from Performance Bike and also a long sleeve heat gear shirt from Under Armour.
In the blazing sun, I also wear a shemagh around my neck to protect my skin and obviously a hat.