Too bad they didn't appoint Ben Hur! We could then have a chariot race featuring him, Jack Smith (the special counsel investigating Trump), Biden, and Trump.
On a slightly more serious note, I think the appointment is well-justified. For obvious reasons, ordinary DOJ employees have a huge conflict of interest in investigating a sitting president. The appointment of Smith was also justified.
For a variety of reasons noted by others (most notably that he didn't try to hold on to the files when discovered, and immediately turned them over to the National Archives), Biden's behaviour seems less egregious than Trump's. But that does NOT mean he is blameless, or even that he should not be prosecuted. Presidents should aspire to more than just being less bad than Trump!
One obstacle to prosecuting Biden - if he does deserve it - is the Justice Department rule against prosecuting a sitting president. You may recall the GOP making a big deal of this when Trump was being investigated, and it is a key reason why Mueller made no recommendation on prosecution in his report on collusion.
Ultimately, my view is that the thing Trump most deserves prosecution for is his effort to overturn the 2020 election. There is no Biden analogue to THAT, despite Biden's many sins of other kinds. But if they both get prosecuted over illegally taking classified documents - well, I won't shed too many tears. Perhaps a few crocodile tears....
Ultimately, if an ordinary former official who did what Trump and Biden did should be prosecuted, the same goes for an ex-president or VP, unless you buy Trump's absurd arguments that "executive privilege" allows him to hold on to the documents even after he leaves office, or that he could declassify them just by thinking about it, without going through any kind of process.
In the words of another Ben Hur character, "it goes on..., it goes on. The race is not over."