Is it preferable to cite the dissertation or a publication based on the dissertation? I'm currently in the process of writing a literature review for a methodology and I came across two relevant works by the same author. The first is their dissertation which includes expanded details about their research questions the methodology they developed to address the research questions. The second work is a publication that is based upon one research question and doesn't contain as much detail about they methodology. My instinct is to cite both,


  method is used by Author et al. (YYYY) in an evaluation of system, which is described in more detail in (Author YYYY).


but generally it seems like only the publication is cited if available. Is it preferable to cite the dissertation or a publication based on the dissertation, or both?
If there is a published version of the paper, you usually want to cite that first. The main reason is that it's the version you're most sure has undergone rigorous review and revisions. As a bonus it will often be easier to find (e.g. through DOI numbers), and citations for papers "count more" so it's nice for the author(s) too. The latter two points should be in favor of also preferring citing preprint versions instead of dissertations, if that's all that's available, but it's a less clear-cut case.

However, if you find that the dissertation provides particularly helpful exposition or, as is often the case, contains information or details not published elsewhere, absolutely feel free to go ahead and cite both! (If you specifically make use of that information you clearly need to cite the dissertation.)