"Requestor" should be reserved for use when implying a
professional occupation in requesting, which is a person who has a
job or an employment in requesting. Whereas, "requester" is anybody
that requests, and not exclusively used for a person with a
profession in requesting. You could think of it like this, a
"Requestor" is an expert in requesting, and a "requester" is not an
expert in the act of requesting.