Electronic Light Switch - Relay or Thyristor?

Wot lyn said.

A couple of small caveats:

An SCR "leaks" current when Off. Not much (milliamps), but occasionally it's enough to fool a some Digital Multi Meters. You need to be aware of it in case you are troubleshooting with a very sensitive DMM, because you may think it is On when it is not. A Relay is an air-gap device, no leakage.

An SCR requires being "gated" on continuously; it technically would turn itself Off every time the current passing through it crosses zero on the Sine Wave. For that reason you can detect, with sensitive enough equiment, a little tiny bit of electical noise in the circuit. Not at all a big deal for the lights, but if you have sensitive audio equipment without proper filtering, you can sometimes detect this as an annoying "fuzz" in the output.

All units of this type that use four-layer devices such as SCR's and Triacs for power switching and dimming will produce some radio frequency interference. Generally, the newer and more expensive ones generate less rfi because they utilize IC's that contain zero-crossing circuits, and incorporate better filtering. The older and cheaper ones don't.

I'd go for SCRs, you will need two of them with separate gating, or one TRIAC, this way you'll have a little electrical noise, but consume less current than the coil of a relay and won't hear any mechanical clicks.

Regards

Many thanks all. RFI and noise not an issue in this application. SCR it is.

Just for incandescent bulbs, here's a good primer/reference that helps explain why an SCR is preferred over a relay. Aside from reliability the bulbs will, theoretically last longer as SCR switching is typically done at zero-crossing in the AC waveform.