Relay Outputs

Higher current carrying capacity. Can be used with wide voltage levels and any polarity. For example, you can use DC 24V, DC 180V, AC 230V etc. The relay's contacts are electricaly isolated from the PLC. 

Relay outputs switch slowly - it will take some 50 to 100 milliseconds for a relay to change stage. This is due to the time required to magnetise the relay's coil and pull the relay's poles. While turning off, the relay coil has to demagnatise and the spring pushes / pulls the poles open.

If the load being switched does not require high speeds, as is the case with most industrial loads, relays are well suited for the job. Example of such loads are motor contactors, lights, most solenoids etc. For loads that need high frequency switching, relays are not suitable.

Relays have limited electrical life -. This makes it unsuitable for loads that have very high switching factor. For example, certaing solenoid coils may have to be switched 50 to 60 times per minute.

Transistor Outputs

Fast switching, high frequency, polarity and current carrying capacity limited at design time.

NPN / PNP