I study petroleum engineering; however, I want everybody from different fields to express their opinion based on their personal experience.
Let's assume you have the following options: publishing in a journal that is not open access and has a high impact factor, publishing in a conference, and finally publishing in a journal that is open access and has a low impact factor. The only option that has ever been appealing to me is to publish in a reputable journal, simply because I don't think the other two are challenging enough and also the whole process of modifying your paper based on peer reviews will help improve the quality of your paper. However, I have recently stumbled upon some papers that had me rethink about the whole thing.
It seems to me as if for some reason open access publishing increases the potential number of citations. The straightforward explanation is that many more people will have the chance to see your paper and there is a huge chance that some of them cite your paper at least in their literature review. (I understand that if they don't have access to high quality journals, they might not be currently in the right setting to necessarily publish a good work, but in this context a citation is a citation even if it's by a poor quality paper)
Also, I do have a feeling that depending on the field you're working in, the number of conference publications can hugely exceed the number of journal publications. For example, petroleum engineering is extremely industrialized and a huge number of fellows in the industry are only willing to publish in conferences. This is simply because publication in a conference is easier, it is fun to travel for free (!) and in general networking is very important to many of them. Also, I keep thinking that there a chance that some of these people, for some reason that I don't know of, have exclusive access to conference papers, but no access to journal papers or maybe they simply don't have the willingness to spend time on reading journal papers because they don't need to! What if my paper gets exposed to this huge number of people if I publish it in a conference, but only to a small number of researchers in the academia if I publish it in a journal.