[AccessD] Number vs text data type

I inherited a budget number database with the primary key being a 7-digit number.  The data type for the field is currently number (long integer).  Since the field requires no calculation, just some sorting, I'm considering changing the data type to text.  The budget number table links to quite a few other databases, so I'll have to change the field in other places.  Is there a benefit to or problem with using one or the other data type?  I remember an instructor once saying that if there's a choice between using a text or number type, use number since numbers sort faster than text, but I'm not so sure he knew what he was talking about.

TIA,
Cindy Spell