Visual OTTHYMO
Visual OTTHYMO is a primarily event-based hydrologic model. It is widely used to estimate runoff hydrographs and peak flows from design storms for development sites and watersheds, which is often what approval criteria ask for.
PCSWMM
PCSWMM is an interface for the EPA SWMM engine. It models hydrology and the hydraulics of storm sewer networks together, including surcharging, backwater, LID features, continuous simulation and 2D overland flow. That makes it strong for sewer capacity and dual drainage questions.
How they fit together
Many projects use both: OTTHYMO for site or catchment hydrology and peak-flow targets, and PCSWMM to test how a sewer and overland system behaves. The choice depends on the question being asked.
Check what the agency accepts
Municipalities and Conservation Authorities often state which models and design storms they accept. Confirm this before building a model, so the work can be reviewed without rework.