I haven’t used either one and I am not an expert here; I know what has worked for me and why I used them (same logic as @Lonster)
“Secondary poisoning risks to predators and scavengers from zinc phosphide exposure are low especially when compared to other rodenticides”
If the goal is to minimize off target mortality, both seem better than other rodenticides. both seem effective.
what is different with zinc phosphide is the risk to humans - “low” when the risk from diphacinone is essentially zero for me. The USDA document cited 25 people are known to have died from zinc phosphine exposure.
and there are “sub-lethal” tissue damaging effects
It seems like zinc phosphide has more risk to the people applying it than to non-target animals. It breaks down slowly into phosphine gas even when it is dry (and breaks down much faster when wet) and you can breathe it in or have dermal contact while applying it.
Be careful where it is stored (it breaks down slowly into phosphine gas) and when handling it (respirator and thick gloves, etc). there are also “sub-lethal” effects from exposure (i.e., it can cause tissue damage in small amounts but not kill.) There is no treatment for exposure.
diphacinone doesn’t have the same risk to me when I handle it. And, if by chance, there is massive exposure, I have several days (3-5) of clotting factors before any bleeding problems and Vit K is an immediate treatment (and cure)
it is always a cost benefit analysis