Terra cotta is cooler as it breathes. Plastic pots in your zone can literally bake roots in the sun. Ask how I know!

Jane

In part, it depends on what color the plastic pot is, and how moist the soil in the terra-cotta pot is. Generally speaking, terra cotta makes for lower root temps, but the advantage starts to wane as the soil moves to the drier side. Another significant advantage terra-cotta has over plastic and many other types of containers is gas exchange. The gas-permeable walls of terra cotta allow for escape of CO2, methane (a by-product of the composting process), and sulfurous gases to be exchanged for air from outside the pot, a distinct advantage. In the end, terra-cotta offers better opportunity for plants to realize their genetic potential.

Al