the two series appear to be good. There is some variation even between chapters in the same book because the chapters are written by different authors and some tend to be better than others in simplifying things for a junior resident to understand. If i were to choose one over the other, maybe the Core is a better pick. If someone thinks The essentials is preferable, I would not argue with that because the two series can be quite similiar. I would recommend getting the whole series at an early stage as they are a reasonable read for a junior resident to learn basics which can be difficult to learn from the more complex books out there.