Well, let me try squeeze my wisdom into this brief reply:
Round roofs offer themselves ideally for a reciprocal one. The big giant PRO is lots of space inside without needing a central column. The drawbacks are the inevitable hole in the middle, and the placing of your rafters. Though in your Arizona climate a vent could come in really handy for air flow, especially if you install some cooling tubes in your wall. Also, the hole will make a sweet skylight, but you gotta build it well: It should keep all moisture out, be easily operable from floor level, and let the sunlight shine through. As for placing the rafters (or vigas), I can only say: with a small team of folks (9 in our case) open to experimenting with "Stonehenge-type techniques" you can do incredible things. But here you really have to consider the length and thickness of the vigas, in relation to the space you want to cover. For our theater building the inside floor space is 10 meters (30 ft) in diameter, and used 13 vigas of 7 meters length. So here you really need to look at what you have and what you want... Please feel free to look through my posts, and I'll be happy to give you more details.