For some people, I think it would be easier to get into an MSTP program than an MD-only program. Typically, these are people who are more interested in research and science and do not have as much clinical experience as other people do.
That said there are less spots available. Will it harm your ability to get into plain medical school? A lot of places allow you apply simultaneously, so the answer would be probably not. The question should be what do you actually want to do? Do you want to be a practicing physician or medical researcher? If a medical researcher, what kind (more clinical vs more basic science)?
If you are more clinically inclined, then MD-only might be the way to go. If you are really into the research though (clinical or basic science), then you should look into MD/PhD programs.