Depends on where you want to work in the long run. The way I understand it, to get upper level jobs with the CDC, NIH, UN, orw WHO, either degree is necessary. HOWEVER---I believe a MPH is necessary if you do NOT have a nursing degree (at least a BSN).
Although, there are many interdisciplinary aspects added to most MSN curricula, an MSN is a nursing degree unlike the MPH . MPH programs welcome candidates from multidisciplinary fields. You receive well-rounded training in biostatistics, epidemiology, environmental health, social sciences, and management. An MPH degree can open many doors in healthcare research and can further one's goal to advance in the management ladder.
Do you want to advance in the corporate ladder as a manager? You still have the option of pursuing an MSN in this case. Some MSN programs are geared towards nurse managers and provides you advanced training in nursing service administration.
In making your decision, ask yourself what you enjoy the most. Do you like staying in clinical practice? Do you enjoy teaching/training nurses? Do you see yourself participating in research in the nursing field? Then maybe an MSN will be right for you.
Also, found this link that may be of interest: http://www.jefferson.edu/jchp/nursing/JSN-MSN-MPH.pdf It's a dual degree.