While there may be weirder ends between the two events as you say, does it make it any less illegal what Biden did?
Nope, the two circumstances have almost nothing in common except that classified docs were in a place where they didn't belong and that is a problem in both cases
That is why the National Archives referred this (Bidens situation) to the DOJ and the AG assigned a US attorney (who was appointed by Trump) to start an investigation
The difference is that you have to prove intent. Did the person who took the docs know they didn't belong to him/he didn't have legal access to them and he intended to retain them
We know this for a fact with Trump starting with his attorney's telling him before he left the White House that he couldn't take various docs. Then we have the National Archives making multiple requests, including trips to Trumps home, to retrieve the documents. If Trump has just given them all the docs at any of those times we'd probably never even know about it. Instead Trump and his agents lied and obstructed repeatedly and refused to return the docs which at that point he could not deny knowing they didn't belong to him (INTENT). He also made many public and online statements admitting he took them and insisting they belonged to him, which of course they didn't (more INTENT). He also obstructed the investigation and the return of the docs (more INTENT)
Let's compare that to the Biden docs. They were found by Bidens attorney's in an office they were vacating and his attorney's immediately contacted the National Archives who retrieved the docs the next day. Kind of hard to prove intent when Biden's attorneys discovered the docs and immediately notified the National Archives and turned them over the next day.
There is are many other differences but that's the most signfiicant
Also, of course it's being investigated. If the US Attorney thinks there are more docs or that Biden is not cooperating he can do what the US Attorney investigating Trump did. He can issue or subpoena for the docs and if Biden and his attorney's don't comply or lie and obstruct then they can go to a judge a get a search warrants. Of course none of that is likely to happen (not just because Biden is POTUS) but because Biden's team is the one who informed the National Archives about the docs are turned them over right away. If Trump had just done that or merely replied iwth any of the initial requests he would not be in legal jeopardy today