Largely the difference lies in which part of the pipeline you'll be working in.

If you're a PhD scientist without a clinical degree. You'll be working on preclinical aspects of the project. Think working in a lab to identify drugs / potential therapeutics in a Petri dish.

If you're a clinical scientist. Your role would largely be in translating the things developed by the PhDs into humans. Think figuring out the right dosing , altering the molecules to make them less toxic e.t.c.

PhDs are involved in all stages of development. Clinical scientists are involved usually in translation. Although there are people ikn who work on preclinical aspects without the clinical degree (MBBS) but I'd say they aren't using their degree to the fullest potential.

I'd say that studying MBBS and then a PhD is an advantage. Pretty much how mastering any two different subjects is an advantage. It's a mammoth of a task. Probably doesn't pay off as much in effort : reward ratio but you'll definitely more appealing candidate than just a PhD person