If you want to primarily become a scientist, I believe that you should definitely either do MD-only or MD-PhD. Getting a PhD only is going to be a very rough course.
When you have a just a PhD, you generally are not hired in a clinical department, e.g. you are hired into molecular oncology instead of clinical oncology. There are several consequences of that:
1) You cannot generate revenue outside of grants. With an MD, you can always practice medicine and generate quite a bit of money for the department
2) Your department probably cannot siphon off clinical revenues to help fund the department. This is an issue when you start up your lab or if you fall upon hard times
As a consequence, if you were a department chair looking to hire someone, why would you hire a PhD only? First of all, there are too many of them (and thus their value is much less) and secondly, they are 100% beholden to fickle grants.
Also, consider when you do your training. As an MD, you do your postdoc as part of a fellowship and then get promoted into a clinical instructor. You are probably going to be one of maybe 3-10 fellows in that program. What happens if you run into trouble? Your fellowship director will have your back. As a PhD, you have no fellowship director. If you run into trouble, you are up a creek without a paddle. It happens. You also get paid a lot more as a clinical fellow in the lab as opposed to a postdoc.
If you absolutely despise clinical medicine and cannot fathom doing pathology, then get a PhD. It is do-able but is rough, and you only have about a 25% chance of getting a tenure track position. Otherwise, stay far, far away.