I do the same (I have both review a piece of code), and Codex tend to produce more nitpicky feedback. Opus usually agrees with it on around half the feedback, but says that the other half is too nitpicky to implement. I generally agree with Opus' assessment, and do agree that Codex nitpicks a lot.

I can't even use Codex for planning because it goes down deep design rabbit holes, whereas Opus is great at staying at the proper, high level.