Hexayurt all the way - we slept until at least noon most days, and didn't have a/c or a swamp cooler, just two window vents. Afternoon sleeping was a little warm, but still bearable (not even an option with a tent), we are adding a swamp cooler this year for afternoon coolness. JK is right about no heavy rugs, I put down only a light canvas tarp on top of the bottom tarp just so the floor wasn't sticky - the ground is noticably cooler, especially after being shaded all week.
Really, the ideal set up (for me) is a hexayurt with a roomy shade structure off the front door of the yurt. When it gets hot in the afternoon, if the yurt isn't cool enough for me, I'd rather nap in the shade structure where I can at least catch a breeze. If I had to tent it, I would probably opt for a big-ass breezy shade structure with a cot or hammock for afternoon naps, and a small tent for private early morning sleeping and possible dust storm protection, but if you are considering a hexayurt, the dust-free benefit of a hexayurt is almost as important to me as the coolness of it.