You need to study harder and longer to become a doctor.
I'm guessing that nurses are constantly on their feet which would probably make it harder. But I'm not sure so I'll say doctor.
Doctor is because you need a higher degree and more years of school and training that a nurse does.
A nurse's training takes less years but you has more experience on the field.
Anyone that willingly empties bed pans and cleans up the patients who have died and other icky jobs, plus puts up with whatever the doctors throw at them has my profound respect.
Training to be a doctor may be grueling, but when you're a nurse it's always hard. I worked with the nurses when my mother was in the recovery room for nine days after open heart surgery. Yep, I said nine. My mother was my only concern but I saw how hard nurses were working with the other patients.