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The Elements of Computing Systems is a project-based course, starting with transistors going through building a computer and then an OS and software on top of it. Code goes deeper into the history leading to modern computers, but doesn't offer this project-based experience from the transistor level up. IMO, the two books offer a nice complement to each other.

As a reader of both books, I couldn't agree with you more.

I read CODE before reading The Elements of Computing Systems. I felt like the former provides details that you actually implement in the latter. Highly recommend both and in that order.

"Elements of Computing Systems" is more detailed and involves you actually creating (virtually) a working computer starting from NAND gates. CODE is much more introductory.

Petzold's Code is a popularization aimed at non-technical readers, as you said. ECS is a textbook aimed at college-level CS and Comp. Eng. students and goes into sufficient technical depth for the reader to build their own digital computing systems.