Both are in dictionaries. I’ve heard people insist “requester” is correct for a person who requests something, and that “requestor” is wrong there, leaving me to wonder how it is used. Requestor happens to be marked as an error in my browser’s spellchecker.
COCA for requester/requestor indicates that requester is used for people and requestor is used in technical senses like a part of a program. A google ngram for requester/requestor shows that requester is also a bit more common.
Some online discussions suggest requestor may have a meaning in law or be more common there, but I can’t find that.
What’s the difference in usage? Should requester be used for people, or is it more nuanced than that?
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.