I can't decide between getting my MPH or jumping into a PhD program in Human Genetics (which all of my background is in). Which will offer me better job prospects? I am aware the fields are very different.
If you're considering a PhD in genetics, why not consider a PhD in genetic epidemiology if public health is a field you're interested in. That is a comparison that's easier to wrap your head around: either degree would serve you very well and job applications would be very similar (academic, industry, government).
It's hard to wrap the entirety of this discussion completely because the MPH vs. PhD (any basic sciences) is very different. What you do the MPH in dictates what your future job prospects are. Of course, if you're interested in a PhD, you're probably interested in being a PI or leader researcher of some sort, which is something you can't do with just the MPH.