There are many articles about the difference between custom taxonomies and post meta data. Here are some differences I experienced:
- There is no index on the post meta value.
- With the builtin WP_Query functionality, you cannot search with operators like <, >, =, >=, BETWEEN, etc. in custom taxonomies, so if your values are numbers, for example years, and you want to search for every post earlier than some year, you have to use post meta data.
- No ordering of custom taxonomy in the admin.
- Post meta data is linked to 1 post, custom taxonomies are shared between multiple posts, because the term_id is linked to the post and not the key and value.