Should I wear cotton or synthetics in Egypt near summer?

I see everywhere advice to wear cotton shirts (long sleeved?) and pants in Egypt due to the hot weather. This is against the "general wisdom" of backpacking, that says to use light clothes that dry fast and can be hand washed easily.

I will spend 16 days backpacking in Egypt (Cairo, Luxor, Aswan, Western Desert, Alexandria and maybe Hurghada or some other beach for a couple of days) during May. Since I will be moving a lot, including tours to the desert, I want to keep the carried weight to a minimum (I will also carry about half my daughter's stuff) and having the choice to hand wash my stuff.

Are cotton t-shirts really worth carrying? Also, considering the sun is it better to have long sleeved t-shirts or short sleeves with some light jacket on top?

A similar question was asked in The Great Outdoors Stack Exchange. The short answer is

Long sleeve light colored thin fabric cotton is leagues better than any synthetic.

The short explanation is that cotton retains water (sweat in your case), so it dries slower. Cotton will keep you cool by slowing down the evaporation experienced at high heat and low humidity.

As to why use long sleeves, that came up too in another question and it gives 3 reasons to be covered from head to toe in a desert:

• Protection from the sun's harmful rays.
• Air flow for convection cooling
• Moisture retention (you heard that right) for evaporative cooling.