Trump is somewhat more problematic. He obtained those documents as President of the United States. As such he had the power to declassify them on essentially on a whim. Biden wasn't President at the time and had no such authority. In his case, he had no such authority.
It doesn't excuse either but it makes Biden's case far more clearcut that he had unauthorized classified documents in his possession.
"As such he had the power to declassify them on essentially on a whim." <--- this is just not true. There is a process to declassify materials and twump didn't do it.