Comparing a career in medicine vs. medical research?
As a person who faced a similar dilemma a few years ago, I think there are major practical differences:


The working environment. Every doctor will spend his early career in a hospital, whether he plans on practising in the community later or not. A hospital environment is really different than the research one. In the former you are constantly on your feet, working long hours, nights, etc. However, you are constantly working with the people to whom you're helping. In a "lucky" week (depends on the POV), you might save a life every day. 
Taking work home. While a M.D works long and hard hours at his/er practice, when she goes home - she can rest. Her patients doesn't follow her home. As a researcher I can tell you that I'm even dreaming on my research. 
Independence. The medical environment is a very hierarchical one. Starting out as a M.D, you can't even sign off your own orders (in first world countries, at least :) ), contrary to a graduate student who might be given an independent project very early in his or her study course.
Goals. As a researcher, you end up working for the sake of science. You might find yourself pursuing a direction which might take decades before it translates into helping people. As a practitioner - you're working for the people in front of you.
Demand for your skills. In general - there are hardly any M.Ds who go unemployed. Most healthcare systems are understaffed and there is always a demand for M.Ds. I wish we could say the same about positions in the academy, and the industry is never as stable (and here it's field specific and I can't say much about the specific field in question).