ACF vs Taxonomies

MHO is that you should stay away from ACF as much as possible, but putting this a side your question is really what is better to use post meta fields or taxonomies, for which the answer will depend on other factors.

For example do you want to easily see in the value as a filter to seeing all relevant post on admin side, or at the front-end then taxonomies are a good way to reduce the amount of required work, but if not, there is just no point in cluttering your admin with taxonomy menus which will never be used.