I wouldn't say that this is best practices, but here are some "differences" that would be your consideration points when making the design decision:

• selection is a fixed list once rendered (this also applies to many2one fields which is viewed with widget="selection"), you cannot apply domain to it
• selection will be pulled altogether at once when HTML is rendered (this also applies to many2one fields which is viewed with widget="selection").  Not a good choice if you have hundreds or thousands selection.
• selection fields (this does NOT apply to many2one fields viewed with widget="selection") is not stored as a Foreign Key.  So you can remove an entry from the selection and no integrity problem will ensue.

As for the states, you can still make it a selection field whilst making it dynamic by supplying the entries from a table.  Selection field can accept a function as selection source.