The title is misleading mainly because there is not much room to write a detailed question.
I am currently working on a (Maths) paper. While doing the literature review, I first found something interesting that I would like to include in my paper in another paper (A) from a not-indexed journal. The researchers who wrote the paper come from a no-name university.
Later, I found somewhat the same thing in another paper (B) from another journal which is well-reputed in my field. (B) is better written than (A). The paper (B) was published before (A). However, I can not tell whether or not A plagiarised B.
I decided to cite only the paper (B) in my paper because of the reputation of the journal/researchers.
Is that unethical and disrespectful to the researchers who wrote the paper (A)?