Schools of Medicine get NIH grants because they have preventative medicine departments (which is public health). USN uses a ranking system of surveys/peer assessment to rank schools.
You want to know if its preferable to do a epi PhD at a SPH or a SoM? The choice should be obvious, a SPH since epi is a PH discipline and SPH would have far more resources and training devoted to it than a SoM would. Besides, not all SoM offer a PhD in epi.