Have there been studies of differences between journal paper authors and conference paper authors?
My relatively quick Google and Google Advanced searches didn’t identify empirical studies that address the question you ask. However, a search using the terms [scientific conference presentation] and [diversity] yielded this article discussing the need for actively working to increase diversity among conference presenters:
https://physicsworld.com/a/fifteen-tips-to-make-scientific-conferences-more-welcoming-for-everyone/
This article does not address your question directly, but it makes me think that framing your question in terms of a comparison between authors of published papers and authors of conference papers may not be the most productive approach to finding relevant literature.
The majority of researchers who have published in peer-reviewed journals have also presented their work (sometimes the same work) at conferences. By making the author/presenter the unit of analysis, one risks substantial overlap in the two groups—-very likely washing out any real differences that exist. “Noisy” data are problematic in their potential to overlook effects as well as to suggest them erroneously.
I think that a reasonable answer to your question would be more likely to come from separate examination of the two avenues (journals vs presentations)for the diversity of their authors.
In 20+ years’ experience with both conference presentations and peer-reviewed publications, I note barriers to diversity in both venues. It is certainly true that attending a conference takes time and money that many researchers lack. However, research of sufficiently high quality to obtain inclusion in peer-reviewed journals typically comes from well-funded labs to which only a small percentage of the academic world has access. Biases in admitting students into graduate programs, biases in hiring academic/research personnel, and biases in the awarding of research grants guarantee that both journal and conference avenues will have less than optimal diversity. I think that separate investigations of the two would be very interesting.