one-to-one schema design question and ORM

I'm developing a system using Ruby on Rails (with ActiveRecord) and
postgres.  (Although I think my question is still relevant for, say,
java with hibernate.)

I have two classes (tables): users and employees.   A user is an account
that can logon to the system, while an employee is...umm...an employee.

When someone is logged in, they will want to run queries like, "give me
a list of my accounts".  This means I need to link the users table with
the employees table.
 From a business rules perspective:
   Some users are not employees (like an admin user)
   Some employees are not users

 I can think of two ways to do this:

1) a 1-1 relationship where the user table contains a FK to the employee
table.  Since not all users will be employees, the FK will sometimes be
null.
In rails, the user class would "belong_to employee" while employee
"has_one user".

2) Create a link table that has FKs to both the user and employee
table.  This make sense because I'm not sure that the concept of "there
might be a linked employee" belongs in the user table.  This moves it to
a separate table designed for that purpose.  But then again, it may just
be a needless extra table.

Would you prefer one solution over the other?