Select field descriptive or numeric keys
The key|label format is for scenarios where you want to present a different, user-friendly, label that differs from the underlying, machine-readable, storage. For example, a country select list that you want represent in the 2-character ISO 3166-1 format while showing the official country names to present to the user selecting it.

In the above scenario, regardless of using the key|label format or not, the underlying data will be represented as a string (VARCHAR) since it's a Field (text) field type. If your underlying data needs to be represented as numeric, then you should use the List (Integer) (or other number types) field type as it will be stored as INT within the DB column.