Electromechanical Relays (EMR) and Solid State Relays (SSR) are very different parts with many differences in characteristics. Here is a comparison table for general cases to help you figure out which type of relay is right for your project.
|Sensitivity to withstand misuse or misapplication||Good||Poor|
|Sensitive to corrosion, oxidation, or contaminates||Yes||No|
|Sensitive to shock, vibration or acceleration||Yes||No|
|Sensitivity to radiation||Fair||Poor|
|Cost per pole||Best||Fair|
|Input TTL & CMOS (buffer) compatibility||Fair||Best|
|Operate and release time||around 5-20 ms||around 0.25-10 ms|
|Compatibility of military/aerospace specs.||Good||Poor|
|Ease of troubleshooting||Good||Poor|
|Input to output isolation capability||around 4Kv||>4Kv|
|Normal failure mode (output)||Open||Shorted|
|Normal wearout mechanism||Contacts||LED|
To learn more about how electromechanical and solid state relays compare in output switching capability, please refer to TE application notes on Relay Technologies.