I personally would go (and have gone) the RA route. The short is that an MA is going to be much more expensive (net out the tuition you'd pay against the salary you'd earn as an RA), while the outcomes are likely to be similar; if anything, an RA job may be more beneficial for you given you don't have research experience so far. Sort of a side note, but some RA jobs will allow you to take courses while working - either through tuition reimbursement for a nearby school or online course, or at the school you might be affiliated with in your position. If you're concerned about your Calc grades, you might consider taking a course in Calc or Analysis while working as an RA and aiming for a better grade.
One tricky thing with RA jobs is that a huge selling point in applications is previous RA work - it's sort of a catch 22. Prior RA work shows that you can do the work at the job you're applying for. If there's any way you can get some sort of research work going over the winter or spring, that would help RA applications quite a bit. If you really can't get in with the professors at your school, perhaps you can take a course that will have a required research paper assignment. Or, if all else fails, spend a little time drafting a brief research memo on something you're interested in - e.g. download some CPS data, play around with it, run a few regressions and write about what you find. Having some small empirical work you can point to will help in RA applications. If you can't RA with anyone, I'd make a point to list any research projects (in courses or even a short independent project) on your CV when applying to RA jobs. In their quick first round screening of applications, reviewers will want to see that you have some/any level of experience.
All this said, you should make your decision based on your own profile and goals. I saw your other post that mentioned you're interested in programs such as NC State and Clemson. I'm not as familiar with these programs, and it's possible that the MA might make more sense in light of this. For example, it might be that these programs are willing to forgive lack of research experience but do want to be certain you can handle the course material. You might spend some time looking over the CV's of current students at the programs you hope to attend, and getting a sense for the typical profile of a student in these programs. This can help clarify whether you stand to benefit more by focusing on addressing grades or research experience.