"Both would be valued in Africa; if you can do both, then that would be best."
In the part of Africa that I'm familiar with, both a nurse midwife and nurse practitioner are extremely valuable professions - if you don't mind how you are licensed. Nurse Training Schools in Africa most frequently triple qualify their registered nurses. When they complete their training they are qualified as a general nurse, a community nurse, and a midwife. Diagnosing and treating is included in their basic training so they can be posted to work on their own or with a clinical officer (similar to a PA) to run outlying clinics or even a health center (better equipped than a clinic, and do bigger things like deliveries).
In my opinion, either qualification would be valued but I might place midwifery a little ahead in importance. Your personal preference would be important as I truly mean that both would be valued in Africa. If you are ambivalent, then lean towards midwifery, as pregnancy and all that surrounds it is a huge healthcare issue for Africans. If you can do both, then that would be best.
While I realize that spending more years in training keeps you from sharing your knowledge and skills with people who desperately need them, healthcare in Africa is just so much more challenging - usually because disease/care issues are so much more advanced when they present at the clinic. I personally do not know any medical missionary who would say they had over-trained. I can name many who wish they had more training.
The Center for Medical Missions produces an e-newsletter every other month with the purpose of walking beside those who are preparing to serve long-term in medical missions. If you think that is something you might be interested in, you can sign up online. All the old issues of "Your Call" can be found there as well.