It's not a two tiered justice system, and historically you wouldn't be imprisoned unless you were, or had intent to, show/communicate the documents to others who do not have clearance to view them. Those who have been prosecuted received fines, probation, loss of clearance; and if they communicated the documents to others then probation or extensive jail time depending if it was espionage or not.
There is also a massive difference in handing over the documents before the authorities inquire about them, and willfully keeping them when asked to hand them over (just like there is in the lower court systems and plea bargaining).The later is definitely obstruction of justice when the FBI is involved. Not to mention whether or not the Presidents knew the documents were in their possession in the first place. It's quite clear that Trump knew exactly where the documents were and what they were, Biden maybe not. I'm perfectly ok with Biden, Trump, and Pence receiving fines for possessing the documents. I'm also more than ok with Trump being prosecuted for obstruction of justice.