These are fossils of a fish (vagrant pelagic vertebrate; sample a), and a coral (sessile benthic cnidarian; sample b). in general, which would be more useful in paleobathymetry (estimation of past water depth), and why?

The coral fossil would be a better estimate of past water depth. This is because most corals inhabit shallow water reefs, less than 150 feet deep. In contrast, pelagic fish may inhabit any open water (pelagic) part of the ocean, with depths ranging anywhere from a couple of hundred feet deep to miles in depth.