The fields are extremely different. H/O is an IM subspecialty, which means that you are expected to deal with everything that entails - UTIs, pneumonias, hyponatremia, hypomagnesemia, renal failure, transaminitis, fatigue, hypothyroidism, diabetes management to an extent, blah, blah, blah - in addition to having to master hematology and medical oncology and dealing with chemo toxicities and emergencies, endless calls about pts with hemoglobin 6.5 or platelets of 3k whom you now have to call to recommend ER visit for transfusion (and then they ask whether it can wait until the morning and you're in the pickle of having to choose, in awkward silence, between medicolegal safety vs coming off as a worrywart CYA practitioner), etc. In truth, the field leaves little time for other hobbies or having much of a life. At least that's how it seems as a fellow whose attendings spend their "days off" prepping for patient visits, doing admin work, catching up on notes, research, etc. It is a terrible grind for a "normal" person who wants to have much of a life, IMO. Kind of looking for an eventual path out, TBH.
Whereas when I did my RO rotation in med school, it was a cool combination of clinic and drawing cool 3D simulations or maps on computer screens; basically, medical sci-fi. And I'm pretty sure ROs don't have to stick stethoscopes in their ears (I hate those things!). RO is NOT going to disappear. The geographic flexibility is limited, no question about it. But the oversupply issue will be solved once the academic elites have a collective panic and "oh sh**" moment, which they will have at some point this decade. From there, it's a simple matter of slashing spots by 60-70% for several years to re-establish supply/demand equilibrium.
Of course, the right answer is neither of these. Do Radiology.