If each author can only have 1 country then it really does not matter. If you should want to list all authors in a specify country either a taxonomy or a custom field (single select) will perform just as well.
If on the other hand each author could be listed in multiple countries then you are better off using a taxonomy field (custom taxonomy). Taxonomy queries are easier and perform better than querying for posts based on a multi select field. The reason for this is that a multi select stores values as a serialized array. Looking for one value is not a problem, but looking for posts that might match more than one value becomes complicated and the query performance degrades as you add more values to look for. See section 3 on this page https://www.advancedcustomfields.com/resources/query-posts-custom-fields/
