I'm concerned about both sets of documents, still waiting to see how any ended up in a VP's residence - and how security-critical they might be (lots of documents get classified without being any kind of threat to the nation if released - and that's the legal issue, there is no law about ‘classified’ documents).
Trump might have taken these accidentally, but his behavior afterwards suggests that that was not the case. He had to be pressed to return them, and even then he didn't return all the ones he was aware of. He seemed to be daring the Justice Department to take action.
From his initial actions, I would guess that Biden having the documents was more a mistake than deliberate. Like I said, I'm waiting for more information; he's not automatically innocent because of his party, any more than Trump was automatically guilty. My major concern is around how this happened at all - and how we make sure it doesn't repeat.
Also, I'd expect this to have some bearing on Biden's decision to run for reelection...