I would think carefully about doing the graduate work. You should appreciate that I am somewhat biased as I have discovered that research is not my strong point. With that said, I am not sure what the relevance of graduate work would be to medical school. Why would being a researcher help you out on the wards? Everything you need to know to survive the wards will be taught to you as the majority of the class does not have a graduate degree.
The only answer I can find is the "life experience" you gain by working in the lab. My reply to that is that you would find significantly more life experience by working, and then travelling and volunteering. Graduate work should be done for graduate work's sake, so if you enjoy research, by all means go right ahead. However, 70 people in this year's med class out of
120 got in without anything more than a Bachelors degree. As far as credentials go, publishing is always good. It shows that others have acknowledged your work, and it's certainly something to bring up on your autobiography and interview.