Of course there's a legal difference between the two. For starters, Trump's removal of documents and his obstruction constitute separate charges in the indictment, under different statutes. Obstruction of justice is a crime against the justice system itself, and people have been convicted for obstruction of justice without committing other crimes. Govt. officials mishandle documents all the time, but obstructing an investigation of such mishandling is exceedingly rare—certainly to the extent and with the determination we've seen here.