Either do all MD or all MD/PhD.
Actually up to around last April, I was debating whether to apply MD or MD/PhD. I have around 7 years of research stretching from high school science fair to now, and ~6 pubs/chapters, etc., so it wasn't due to lack of research or a poor application. What led me to my decision?
1. If an MD/PhD were to make the decision to just pursue medicine, then this isn't optimal because the PhD splits the medical education into two segments, and by third year, there will be some difficulty adjusting to rotations (because the last time you've seen preclinical material was around 4 years ago). This is assuming that your PhD will take up most if not all of your time.
2. Besides the cost, there isn't really a difference between getting an MD first and then getting a PhD later (if necessary).
3. There's an issue of matching a PI that you'd perhaps want and the clinical experience from the medical school. In other words, you have to choose the entire package - both the PI and the medical education has to be from the same school. This isn't as flexible because it may limit your options for PhD.
4. It's not really the degrees you get that is important - it is the laboratory training. I think it's definitely possible to get postgraduate training equivalent to a PhD without getting a PhD right now. It's entirely possible to do research with just an MD; the amount of funding you may get is dependent on your ability, not your credentials.
5. I don't know what residency and specialty I am going to have. If I were to do surgery, it's unlikely I will have the same amount of time for research.
6. You're going to spend your twenties in one spot. You'll be in your early to mid thirties before you're ready to start a lab or practice. Just something to consider, especially if you want to support a family.