The main question you should ask yourself is why do you want a Ph.D. If you want to be a professor, it's a requirement. If you want to do research in the pure sciences it's important. If you want to contribute to public policy... it can definitely have an impact. If you just want a satisfying job that pays well and which your excited to do, PhD is just a long detour.
Consider the following:
- A social science Ph.D could leave you competing for a very small number of jobs against all the other recent Ph.D graduates.
- A science/engineering Ph.D could cause you to specialize in ways that aren't very beneficial on the job market (this was the case with me)
- Your job options if using your specialization are narrowed down such that you have few choices as to what region of the country to live, and are competing for those small number of jobs
- If you are specifically focused on medicine, much of the above advice might change
- There is a risk of appearing overqualified if you discover that opportunities that take advantage of your PhD are not acceptable. This can be overcome but recall that you do not get industry experience in that Ph.D program.
- Love of learning is a good reason to pursue this path
But if you just want to get a job in the workforce, the 6-8 years you spend getting that Ph.D are 6-8 years of building a retirement account, getting salary raises, and learning about an industry that you have lost.
Getting a masters degree on the other hand is a good boost to your salary, reputation and expertise. And if on completing the masters you really really want that Ph.D... go ahead. You'll certainly have the opportunity to talk to graduate students and faculty at that time while making your choice.
Finally: having a few years in the workforce gives some Ph.D (and masters) students more perspective on their industry... as well as more savings to get them through the program. For example: I got a Ph.D in Software Engineering, when I had no prior exposure to software engineering. Experiencing software engineering in the real world would have made that a much better experience for me.
If you are pursuing a medical degree, you'll have internships to give you experience in the field, and may help point you towards research that you really want to do in a Ph.D.
Spending time in industry may also help you prioritize whats important to you, so that if you do pursue that Ph.D it will be something you pursue with clearer idea of your goals (see my first paragraph) as well as more clarity on the alternative life you might live without achieving those goals -- the life you'd be living while in industry.