HHO and HCS give similar outputs from different sources.
HHO is the most immediately alluring as it promises to power our vehicles (in part at least) on plain water.
The problem is that it takes power from the engine (by way of the electrical system) in order to run the electrolysis system so the Hydrogen production is necessarily limited.
Hydroxy is highly explosive so safety is a priority with HHO.
HCS cracks Hydrogen from gasoline vapour and it does this without taking any power from the engine.
Therefore the Hydrogen production is technically unlimited.
HCS produces no oxygen which makes it much safer than HHO.
Hydrogen increases the burn efficiency of the engine and adds a powerful 'expansion medium' to the equation.