The MSTP route is primarily for those individuals interested in careers in medicine and basic biomedical research.
It sounds like you are having considerable doubts. I am surprised that you are unsure at this point whether to pursue an MD or MD/PhD. It seems like a decision you would have made much earlier.
The stipend and getting into a better residency are not primary reasons for spending an extra 3-4 years completing a thesis for the PhD. If you cannot see yourself doing basic research, then going through a PhD program is probably not worth it and you will likely be less than motivated to finish. Instead, you might consider going the MD route and taking a year off to do research, especially something along the lines of what you are more interested in (i.e. the surgical subspecialty).
Hope this helps.