Rain gardens rely on natural soils that typically allow water to infiltrate and percolate through the soil at acceptable rates. Above ground, rain gardens look the same as biocells. Below ground, they look different - they feature a natural, healthy soil profile that lets water move down through it. Rain gardens are like biocells except they don't have the fully engineered subgrade.
Rain gardens reduce the volume of runoff. They capture and break down pollutants that typically move in urban runoff to receiving streams. They are cheaper to install because there is less material and labor involved in the installations of rain gardens, due to the good soil quality at a rain garden site.