What's the difference between "requester" and "requestor"?
With one exception 1 I don't think there is a nuanced difference here but rather multiple formations of the same concept. As you noted, requester is more common than requestor.
This explanation about choosing -er or -or includes the following:
4. -or is used when the base word ends in -ate, -ct, -it : calculate/calculator create/creator investigate/investigator contract/contractor reflect/reflector conduct/conductor visit/visitor exhibit/exhibitor edit/editor
1 In programming languages that have the concept of an interface you sometimes see the "-or" form, so follow the conventions of whatever domain you're writing in.