A little of both, I think. While Einstein clearly did not believe in the idea of a "personified" God -- one that was conscious or had "plans" or anything like that -- it also doesn't seem entirely accurate to say that he used "God" as a label for "just nature".
I get the impression that he envisioned some sort of thing underlying nature as we know it, or beyond it in some way, which was not a "being" in any sense of thw word, but which imbued nature as we know it with order and complexity. That which provided our universe with its "spark of magic", if you will, which could be found if we pulled away the curtain of our own universe and looked behind it. Even if it was just a "metauniverse" of a kind, operating by its own supranatural (as opposed to "supernatural") laws which spun off our own, Einstein would consider it the "God" of our universe, the "prime mover", the source which was even more awe-inspiring than our already awe-inspiring "bubble" of a universe within the larger.