I'm sure you're done with school by now but for others looking at this post... If you're going to school for medical physics I would stick with the radiation therapy route if you want to do clinical work. This will give you the most opportunity to work with patients. In a perfect world, you'll be an integral part of the radiology department, consult with the MD and patient about the risks and rewards of therapy options... and even deliver the bad news when radiation therapy doesn't work. Just as most medical physics fields, you'll need to be certified by the American Board of Radiology. This is the most important thing. A PhD is not necessary to practice, unless you want to do research.
Health physics can turn out to be a lot of different things. It will not necessarily require certification with the ABR but will most likely require certification from the ABMP or ABHP as a Certified Medical Health Physicist or Certified Health Physicist, respectively. Either can take you into hospitals, universities, power plants, government facilities, or industry, with a focus on radiation protection and regulation of use (lots of regs!), NOT radiation treatment. Health Physics is not an exact science and most things simply come down to a judgement call. If you go this route you will most likely start out as a specialist and no matter what it will take lots of experience to build the knowledge base you will need to be a successful supervisor, so a PhD will not start you out higher but will give you the opportunity to move up faster than your counterparts (but not by much) and you may get jobs easier. In fact you will spend a surprisingly long time at the same level of us mere mortals with only BSc degrees or people who went to law school. This is mostly due to the heavy emphasis on regulations. There's also not a lot of opportunity for research. If I were starting out in this field now, I would definitely go the masters route. Either way, health physics is a pretty interesting field, sort of a merge between hard science, soft science, and medical practice. You're not a real physicist, physician, engineer, scientist, or safety officer, but you'll need to be a lot of those things and once you're certified you'll be paid pretty well.