I think I can clarify a number of points.
Biophysics does deal a lot with molecules at the sub-cellular level. You might study how EM fields interact with living tissue, protein folding, diffusion/effusion/perfusion, interactions among molecules, etc. Medical physics deals primarily with the physics that governs medical imaging modalities and the physics that how radiation interacts with matter. The latter is applied clinically in the field of radiotherapy. Depending on your biology background you could do well in either. Most medical physics start their training with little to no biology background. I don't know if the same is true of biophysicists.
From what I've read, medical physicists are basically equipment and safety technicians for radiation machinery in hospitals, they design protocols for radiation procedures, and may be involved in the designing of new equipment.
Some medical physicists do perform these duties. However, a large portion of medical physicists are also employed in basic research. They look for ways to improve existing medical imaging technologies or to more accurately determine the dose distribution in radiotherapy patients. This might involve Monte Carlo simulations, phantom measurements, etc. If they are employed clinically in a research hospital it is likely that their duties will include research and publication in addition to their clinical duties. If you only receive a masters degree in medical physics it's likely you will be doing routine tasks like daily QC procedures on equipment, or assisting senior medical physicists. You probably will not spearhead research projects of your own. There is plenty of room in the field for both pure clinical and pure research oriented medical physicists.
Medical physicists are most assuredly NOT safety technicians, although they are often involved in the commissioning and QC of equipment. There are regular technicians that deal with the day to day operations of the equipment. Their knowledge goes above and beyond the basic operations needed to run the equipment. There is a large difference between knowing what button to push to take a picture and understanding the interactions that take place in the body and the detector to form the image, the noise properties of the image, the solid state properties of the detector material, and a whole range of other physics concepts that go into making a medical image. Same goes for radiation therapy.