If no special criteria other than length is specified I usually select varchar (yes, I'm a programmer). However given the fact that the data was to be used as "identifiers", coupled with the relatively short length, I picked char. Indexed or not, the column is likely to be frequently used in where clauses. My instincts say that for those reasons, char will be more efficient.

I get the feeling that John derived this question from a real world application rather than just for kicks.