Solid State vs. Electromechanical Relay
An electromechanical relay has less resistance when switching current (about 100 milliohms for an EMR compared to 10 ohms for a SSR). This means that the surrounding air and enclosure is enough to cool an EMR, while many SSRs require additional heatsinks to protect the circuit.
An EMR is more versatile and rugged since it can switch higher load currents and is a good all-around solution for switching applications.
Electromechanical relays are safer because they tend to fail while the circuit is open, meaning there is no current flow. On the other hand, solid state relays tend to fail while the circuit is closed meaning that a current is flowing unimpeded. There is less risk of electric shock working around a failed EMR versus a failed SSR.
Electromechanical relays have limited lifetime expectancies because all mechanical devices eventually wear out. Since there are no moving parts, solid state relays can potentially work indefinitely; however, the transistors in an SSR are still subject to failure from overheating.
An EMR has relatively slow switch times compared to a solid state relay: about 10-15 milliseconds versus < 1 millisecond (it takes longer to move a mechanical part than to shine an LED and activate a light sensor).
The larger contacts in an EMR means a larger package size compared to a solid state relay.