For me they bring up very different mental pictures. When someone says "mother nature" I really do imagine a female figure, goddess-like - Ceres, I suppose. When someone says "mother earth" I visualise the earth. It's not that they aren't related concepts, because of course they are; it's just that for me, one is more about the personification than the other.
I've never thought of Nature as existing only on Earth. At what level of the atmosphere does it disappear. Is the non-Earth universe, for example the Sun, unnatural?
For me they bring up very different mental pictures. When someone says "mother nature" I really do imagine a female figure, goddess-like - Ceres, I suppose. When someone says "mother earth" I visualise the earth. It's not that they aren't related concepts, because of course they are; it's just that for me, one is more about the personification than the other.