A Silicone Controlled Rectifier (SCR) is mostly used in applications involving medium to high voltage. It is used dimming (ie lamp bulbs) and is a favorite for project that require complete control of power output in a heating application. Another value is that it could extend the life of a heater as it regulates the power output in a more controlled fashion (think about how much damage is incurred with the slamming the brakes of a car. SCRs avoid the “On-Off” power surges with a control release of power).
Solid-State Relays (SSR) is an electronic switch (no moving parts) that turns on and off the process using a small voltage supply. Heat sinks are typically used to dissipate the generated heat in this method.