Mysql: Varchar or number for better performance?

Which one is better for performance as a unique index, varchar or number?

Which one is better for performance as unique index, varchar or number? I have peculiar requirements where my unique key is varchar(6) something like "006500," which is used in many other tables as foreign key. I was wondering which can give me better performance in this case -- defining one numeric unique ID number and using this as foreign key in other tables? Because in the last case i need to make two queries first to retrieve unique ID number and then on other table where it is foreign key to retrieve the value I need, while in the first case I can make query directly to the foreign key table.

In all my years of working with databases, I have never once worried about whether an index on VARCHAR2 or NUMBER columns performs better. If I have a number, and I will be performing some sort of numerical calculations on it, then I would never store this as a character string in the column. And unless I had a specific reason to do otherwise, I would store the value as a NUMBER datatype even if I am not performing calculations on it. Many times I have gone into database design thinking I was not going to perform calculations on a field and then had to change that idea in the future. I would store your data as a NUMBER because that is the proper datatype for the data. But I still am not thinking of performance reasons here. Any performance differences are likely to be so miniscule as to go unnoticed.