It can of course vary considerably by store, and I’ll happily spend time in both but, generally:

Second hand bookshops are curated actively - like, they’ll only stock desirable books. They’re owned and run, usually, by people who love books. The staff tend to be knowledgeable.

Charity book shops are much less curated - to the extent that some just stock whatever is donated (which, of course, is largely made up of books people don’t want), so they tend to have a large collection of random books of not as high quality. They’re run by volunteers - which generally means enthusiastic staff, but it does not mean knowledge about books.