I earned a master's degree in neuroscience before heading into my MD/PhD program, and I have a few thoughts on the matter.
1) If your GPA truly is low, a MS probably isn't the best way to improve your application. In my case, I earned the masters as a way to spend a condensed year and a half constructing, researching, and defending a thesis. I found the research immersion invaluable, and I found that the research aspect was what MD/PhD programs cared about - not my graduate GPA. That being said, I would only suggest a MS if your GPA is already solid, and you can find a program that will allow you to complete a thesis outside of the classroom.
2) If you attempt a MS, attempt it in your field of interest. If you ultimately are accepted to an MD/PhD program at the school that granted you your masters, you have essentially saved yourself a year or two of PhD work. Similarly, many other schools may be willing to exempt you from certain classes or allow you to test out of them. Pursuing a different field for the sake of diversity alone is probably a waste of your time.
3) It can be very difficult to apply to MD/PhD programs while finishing a MS. You will have to miss graduate classes, journal clubs, and seminars - you will have to rearrange your lab schedule, and you will probably annoy your committee at least once. It's not impossible, but you need to be aware of the difficulty before trying it.
4) There are a few other things to consider when it comes to a MS. They are not cheap, and if you are involved with both classwork and a thesis project they can be panic-inducing due to the fixed end-date. They can provide excellent exposure into grant writing, working with a committee, and the process of proposing and defending a thesis, but they are not the best way to improve your GPA or gain research experience for the first time. If you GPA is lacking, try a post bacc. If you want more basic research experience, find a job as a tech and save yourself some cash.