Hey SNJ,
In my opinion I would say write your MCAT. Having your MCAT will allow you to diversify where you apply, gaining more experience in the application and interview process, and increasing the likelihood of gaining admission into Medical School. I have seen far too many people that apply to only one medical school for year after year, becoming frustrated with their lack of success. However, being an RN puts you in a special position as having to wait an extra year (or several) is not as difficult as it may be for applicants without a profession to fall back on.
Although it is 100% possible to get in based on ECs and Casper alone, as I said before MCAT will allow you to apply to more areas and strengthen your application. I'm not a complete fan of Casper as it is difficult to gauge your success as a repeat applicant as there is no feed back. That being said, it is being used by more schools. Make sure you keep involved with your ECs even while working.
If you consider your GPA isn't high enough to be competitive there is the possibility of graduate work to improve your GPA. Many schools provide a small, yet significant, 0.2 addition to your GPA if you have a Masters. Some schools have 1 year masters degrees or online degrees that are beneficial, such as Memorials 1 Year course based Masters of Public Health. Lastly, some schools have an increased value on "Mature" applicants, which is characterized as individuals 25years and older. There are many factors to consider together that make a strong application and it is beneficial to improve as many components of the application as possible.
All the best