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?