The difference between Pence/Biden and Trump is like the difference between a friend who takes stuff from your house without realizing it and then returning it before you ever noticed and a "Friend" who steals from your house and after you noticed it instead tells you that that stuff is his now.
As for how casual it is, I think a lot of it has to deal with the type of classified information it is. Is it 3 year old scheduling information that is long past any point of relevance or is it stuff relating to nuclear secrets or otherwise information that is still relevant to be our security.
As far as intent goes, Trump is screwed while Pence and Biden are in the clear.
As far as the content goes, Trump is even more screwed given he had information so sensitive it wasn't even supposed to be removed from the building it was stored in while with Pence and Biden we still don't know.
Overall, the public honestly shouldn't have known about any of this since the searches are still ongoing with them not letting anyone know till it was all completed to keep any other actors from finding out early and trying to search for them attempting to get them before the government could recover them. Trump screwed that up on his end for his opening his mouth, with Pence and Biden, not sure who in their group is giving the bad idea to let the public know before hand.
I understand the potential public backlash if they leaked it before they announced it, but this should have been kept under tighter security all the way around on this.