I posted some of this in another thread, but I think some of the same points apply here.
I am starting as an M1 this year in an MD/PhD program, and I had some of the same feelings as the OP does. I knew in my heart that I really wanted to do the MD/PhD, but articulating that was hard. It just seemed like the perfect fit for me. I was, however, sort of a non-traditional student; I did all my research in clinical psychology and my extra-curriculars and my minor all involved creative writing and publishing. I also took a year off from school and learned to teach and do Zen meditation. So, grain of salt I guess.
The answer I ultimately came up with was this: My passion is research, but that I want my work to really have clinical applications. I knew I wouldn't be content just working in a lab, helping people abstractly. I wanted to really see the people who have the illnesses I'm researching and see the human side, learn about what their experience is like and what their needs are. And to heal them and work with them, directly, in a more immediate sense. Research and practice are two sides of the same coin to me; both inform one another, both are ways that I can help to heal. (Of course, I had clinical experience and LOVED it and wanted more of it--that should hopefully be true of you as well.)
The tricky thing for interviews is that you have to be able to convince your interviewers that you really want and need BOTH degrees. It is a tricky fence to walk. IMO the more important thing is to make sure they know that you're interested in and dedicated to patient care and clinical work (i.e., you're not just looking for a free MD). I say that because as any good researcher, it should be a piece of cake to talk about your research interests and convince them that you love research--it is a lot harder to convince them that you really want to do patient care. The key thing is to let them know that you want your research to really be clinically relevant, and that you think patient care will be both personally rewarding, and will inform your research.
On the PhD side of things, here is a list I came up with of good reasons to pursue the PhD instead of the MD-only route for research:
1. Funding. All good MD/PhD programs give you free tuition for both degrees, as well as a stipend. This is a real boon if you are considering a research-heavy career, because it is a pay cut vs. being a clinical doctor. The less clinical hours you take, in general, the less money you make. Grants help cover you salary, but they are not generous. So if you end up doing a lot of research, the burden of debt is less manageable.
(however, look into the NIH Loan Repayment Programs, which will help you a lot with your debt if you do research)
2. Contacts. One of the biggest skills in academic research is networking. Medical school will give you few, if any, opportunities to do things like go to research conferences and present papers. Which brings me to:
3. Publishing. Part of what you do in graduate school is do a bunch of research and try to get it published. A successful student should have at least a handful of first-author publications after four or five years, which are necessary for:
4. Grant-writing. As a researcher you pay your own way by writing grants, and it's easier to get grant money if you're already published or have pilot work demonstrating your ability as a researcher and the feasibility of your proposed project.
So in other words, what you get from grad school is four or five sheltered years to really learn the ins and outs of research, to learn to write papers and grants, and to meet the people in your field who will help you get ahead. This puts you at a really distinct advantage over MD-only researchers, at least at the beginning of your research career; MD-only researchers, from what I can tell, have to hit residency or practice and immediately hustle as much as possible, trying to write their first grant, get some publications out, et cetera. You also have to make sure you end up in a position where you have some freedom to do research (like a research residency), and those spots are easier to get if you're an MD/PhD.
Of course, that's not to say there are no opportunities to do research as a medical student or that it's totally hopeless; as one of the other posters pointed out, many MD-onlies have perfectly successful research careers. However, what I discovered when doing my own application research was that MD-only research tends to be much more clinically-focused rather than basic. I think it is a lot harder to start in basic research as an MD-only doctor, because your job comprises clinical work. Again, not impossible, but a distinct advantage to having a PhD.
Best of luck to you! The application process is a stress festival, but it'll all be over before you know it.
