If it is an information about a user (probably displayed on on profile pages, or other user centric places), use user meta.
If it is a way to have a collection of users….. hmmm wordpress sucks in this. Best option is probably to add a relevant role.
Taxonomies in general are geared toward creating collection of content, and users are just not content, therefor while you can force some DB model that will let you use taxonomies, this is not going to be fun at all.