I wouldn't be concerned about if you have a major or minor in psych, particularly given some of the important (and related) learning done in a health science degree. There may be some areas you are less knowledgeable, but that doesn't matter as much. What you mean by 'research experience' is what will ultimately matter for competitiveness. Being in a lab is not sufficient. Almost every applicant to me has worked in a lab. I look at those who have transitioned that lab work into some sort of product (poster, etc). Those who have not are not as competitive and may need a masters. There are a number of reasons that people don't produce products (posters at any level) as an undergrad. One is that they started late, another is that they didn't know they needed it, and a very important third is that they are not at a stage where they can do so. This is why I value it so highly.
I would base your choice only on which more effectively helps you get products. Don't just do 'a masters' to do a masters. Not all of them will ensure you get products, and the bar is higher after a masters for you having more and higher quality products (e.g., manuscript submission, multiple posters including at higher tier conferences). Same with volunteering in a lab. If you work in a lab in a gap year fashion, make sure you are part of products to come out of it and make that conversation a clear and early discussion with the lab PI.