How to decide whether to write a "methods paper" or a "results paper"?
Why not both?

I'm not in your field (and methods versus results papers in my field break on different lines than the ones you describe), but in my field the methods papers can be among the most influential and cited works, but iff they are sufficiently unique and sufficiently generalizable.

If you have a method that fits those categories, and also have data that speaks to a particular result, and there is a tendency to split them in your field, there is nothing wrong with making it into two unique papers.

As an additional caveat: as a student, you should have an academic advisor who is well-positioned to give you advice on things like this. Please consult them before taking too much advice from the Academia.SE folk. We try our best, but we are not your advisor.