Answer: That is a difficult location with the hot sun but limited space. It's important to choose something that is native or desert-adapted, but won't grow out of bounds as you don't want to be constantly pruning, which is time-consuming, unhealthy for the tree, and creates unnecessary green waste for the landfill. Of your two choices, I'd go with the mastic. Another possibility is desert willow (Chilopsis linearis), which is deciduous. It drops its leaves in winter, so would allow the sun through to warm the house at that time of year, but provide shade during the hot months. It also blooms with a lovely pinkish, orchid type flower that attracts hummingbirds.
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