How to decide whether to write a "methods paper" or a "results paper"? I'm a grad student in planetary science, and I'm writing my first academic paper. This will be the first paper from my numerical model. 
My results are timely: recent rumors in my community indicate that my model's predictions may be on the right track. 

Papers in my field often fall in one of (at least) two categories: 


"Methods Papers": These papers aim to completely describe the research methods, equations, data sources, and results in great, reproducible detail. Consequentially, they tend to be long, and results can be buried in the details. 


In the context of numerical modelling, such papers might derive the important equations used. They might also describe the details of their numerical implementation, including the governing equation(s), boundary conditions, and so on. 

"Results Papers": These papers aim to be concise and to the point, which is to highlight the results of their research. Sometimes, such papers are rushed in order to present timely results before competing groups do. The methodology is kept to a minimum so that the results take center-stage. 


In the context of numerical modelling, such papers might cite the method/equations/data/etc. from another paper, perhaps with certain modifications, rather than writing them out explicitly. Only the most important equations would be given. 



How should I decide which approach I should take? What factors should I consider? 

I see this as really asking how to balance the following factors:


Thoroughness — ensuring exact repeatability
Conciseness — so that the reader doesn't have to read a bohemoth, and/or doesn't accidentally skim over the important parts
Highlighting results — emphasizing the model's predictions/implications
Timeliness — getting it published soon


I feel like I can emphasize one, pick two others, and let the remaining factor fall by the wayside. (Did I miss any other factors?)
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.