My choice wasn't based on any really compelling reasons. I shadowed a physician (cardiologist) in college and I thought she was the best physician I had ever met (and happened to be a DO--the first time I had ever heard of osteopathic medicine). She told me about osteopathic medicine and I checked it out while applying to schools.
When it came down to making a decision between MD and DO, the reason I decided on AZCOM was that I felt most comfortable there. I liked the environment, curriculum, faculty and the students. I didn't really know much about the DO "stigma" some people talked about.
The option of having an additional tool for my patients (even if I didn't use it--I would know a lot about it) and the esteem I had for my mentor were underlying motives--but my main motivation was that I liked the school. Like I said, it was nothing too compelling. Hopefully I made the right choice!