Table join vs. function

 Which is more efficient, joining two tables or calling a user-defined function to lookup a column value?

For Example:  select a.id, b.description from table_a a join table_b b on b.rec_id = a.id

                                                              or

                         select a.id, get_b_desc(a.id) from table_a a

The get_b_desc() function accepts an id, performs a select on table_b with the passed in id and returns the description value.