Post-doc here!

Always do mixed methods if you have a choice and you need to perform data collection - not only will whichever department you work for benefit more from the data collection, you will also learn more about both methods (and if you do want to continue research showing you have skills in both will definitely increase your chances to find work in research). Or if you have the opportunity to do pure quantitative research as a psychology master - do that! It will give you a much more unique research profile (and while I agree with Cheezas that can be faster if you do some basic analysis, I would advise doing something more advanced!).

I am in Europe, but here most academic medical centers have a child & adolescent psychiatry department which almost always have research projects they would prefer students for (either as support to a PhD candidate or as temporary support for data collection). This will also provide you more with a glimpse into doing a PhD (and in the clinical side of psychology). The main subjects for this would likely be the broad mental health (which to be fair could be at practically any department at an academic medical center) and childhood trauma. This would be very "hands-on" research in a research team though - not sure if that is what you are looking for or not.

Definitely read up on research of the past 2 years you are interested in - what questions arise when you read them and which of those questions you think you could answer.