Similarly to how a Postdoc researcher is a researcher, I think that a PhD student should be considered a researcher. Especially when:
- A salary is obtained for the research that is done as part of the PhD studies. Per definition, I would say that students pay, they are not paid.
- A research master has been studied before and there are no lessons, classes, exams, etc. So in fact the person is not a student, but "at least" an "apprentice".
I know that naming things in one way or another may not make a big difference for those in academia that read the CV, but it may evoke different ideas for those out of academia that read it.
AFAIK, they are absolutely equivalent, even if one is more used than the other (mistakenly, IMHO). Nevertheless, I would like to get confirmation, if possible from an authoritative source (like a dictionary for titles), and at the same time I think that the question may be useful for many people.