MD/PhD where the science is terrific. Your interests may change with time and you always have a residency/fellowship to do later to more direct your interests. Choose a place that will be supportive and let you grow and learn, rather than picking directly based on what you think you want to do now.
If you're seriously considering the PhD only route, you need to ask yourself if you're really interested in the medical school part. If the MD/PhD thing doesn't quite do it for you, than maybe PhD would be the right thing. If you know you want to be a physician-scientist, I would put the PhD-only route out of your mind.