Mind you, I am in Ohio and don't know beans about the southwest in terms of predators or the quality of the land. However, I'll stick my nose in here anyway.
I am in the city, yet we get more than our share of possums and raccoons -- these animals are scavengers and have learned that people leave behind all sorts of easy pickings -- they'd rather be in the city for a free meal than in the country for the occasional or hard-fought one. I don't know if you have either possums or raccoons in your area, but if you do, they will happily and quickly find your chickens.
Where I am the ground holds a LOT of moisture, so a dirt floor would be a problem for the health of the birds AND the life of their housing. Most barns here have a stone or concrete foundation to keep the wood siding from wicking up moisture. As for the birds, moisture is a required breeding ground for molds, mildews, bacteria of all sorts and probably viruses. Bumble foot and respiratory conditions are two which spring to mind.
If your ground is not moist, if it is easy to keep clean and dry, then in your area a dirt floor would be ideal. Poop could be easily scooped. There are ways to deter predators from digging, such as attaching 8" to 12" of hardware cloth to the outside perimeter of the coop and burying it under a few inches of sand. Electric fencing is another deterrent.
Just be aware of what the choices are and why. And don't think that just because you are in town that you are free of predators. Even if your yard is surrounded with block fencing, one or several coyotes would happily come in to feast on unprotected hens.
Let us know what works.
Jenny
