I think they could do both - save the Ba'ku and get the Meds
Surely the Ba'ku don't take up the entire planet... therefore, Starfleet could establish a Medical facility on the other side of the planet so that sick people could stay for a month or two to regenerate??
But, see that involves the Federation actually talking to the natives and actually questioning the assumption that they give a f@#k about what people on the other side of the planet do and not forgetting that federation medical technology could probably keep the Son'a alive long enough for the rings to affect them.
except that the movie explicitly contradicted that with regards to the Son'a.