How can the choice of "where to publish" affect the potential number of citations?
A prior question would be why you care about the number of citations. There are valid career-related reasons, and I assume that you know that citation-gathering businesses do not always include "self-publication" and conference proceedings. That applies both to the article being cited, and the citing article. So there is a difference between actual citations in the abstract sense, and enumerated citations. That could support the refereed respected journal approach, depending on your motivation. Also, there are numerous open-access respectable peer-reviewed journals, so since you are looking for non-overlapping categories, I assume you specifically mean unrefereed repositories of various kinds, in referring to "open access".

I would be surprised if there is any evidence that open-access results in an increase in citations, because in the subset of people in the world publishing academic papers, I doubt that the factor of having to pay to read an article has a significant effect on whether a researcher can actually read the paper. Via my university, I get to read articles in any arbitrary journal, and request a free copy if they don't subscribe. An opposite expected trend is that an article in a large, open repository may more easily be missed, if say they get a hundred submissions a week, and if authors are unaware of your work, they won't cite it.

In my field (linguistics), conference proceedings were traditionally just a minor notch below peer-reviewed journals in stature, due to there being few journals and a major publication lag, but now conference proceedings are in third place (behind book chapters). But in some sub-areas (computational), conference proceedings are a major publication venue. Correspondingly, CL conference proceedings go through a serious wringer.

So your basic distinction centered around peer-review is correct, it's just that peer-review standards vary considerably for journals and conference proceedings. It may be that more people will have access to your paper if it is in an open repository, but they will also have access to thousands of other papers, and to generate a citation by the subset of the public that writes scientific papers, you would need to do something extra to draw attention to your paper.

I suggest you first scrutinize the notion of "citation", to determine what citation engine(s) you care about, and whether they scoop up citations of un-reviewed works that appear in other un-reviewed works.