Recently, I was joking with a friend about the presence of lawns in Los Angeles. I think I concluded our conversation with a remark to the tune that, “It is utterly ridiculous to have a lawn in Los Angeles.”
A colleague, someone who teaches in the same horticulture program as I do, standing behind me, heard the comment and erupted into a tirade in defense of lawns and ended by declaring, and I think I'm pretty close to exactly what she said, “If we didn't have lawns, Los Angeles would be a hell hole!”
If you have only to choose between lawn and cement, I think lawn wins on being cooler, but concrete... Concrete has it's own sustainability problems and it's not a choice we have to make, so it's a moot question.