“Key difference. Biden returned his. Trump didn’t.”
Not a difference at all. Trump is not being charged with a violation of the Presidential Records Act (which has no criminal penalty anyway). He is being charged under the Espionage Act. Biden clearly could be charged under the same act for unauthorized possession of National Defense Information.
You’re correct on the Presidential Records Act. There are no criminal penalties or an enforcement mechanism associated with this law but other federal laws do punish folks for mishandling government records. For instance, under U.S. Code Title 18, section 2071, someone could face a fine and up to three years in prison. (P)Trump is charged with willfully retaining national defense information, concealing a document in a federal investigation, corruptly concealing those documents, scheming to conceal, false statements and representations, withholding documents, etc. The old coverup is worse than the crime. (P)When discovered, did Biden do any of that?