When it comes to switching to LED lighting for your home or business there are a bewildering number of variables to consider. Color temperature, lumens per watt, bulb shape and fitting and of course the choice between mains versus low voltage electricity.
Many people opt to initially replace halogen spotlights (rather than regular omni-directional bulbs) with LED versions for two very good reasons: first, halogen lamps cost a fortune to run and second, modern LED equivalents are excellent in terms of replicating their light quality.
However, many folk also often have their halogen lamps powered by low voltage transformers. Why? Because without getting too technical about it, the lower voltage is matched by an increased current (Ohms Law) which mandates thicker filaments which in turn burn brighter. In other words, 12v incandescent lamps are more sparkly than their mains powered cousins.
In the world of LED though none of this holds true, for the fundamental reason that the light is not a byproduct of intense heat due to the electrical resistance provided by the metal filaments. Photons of light are directly generated by inducing an electron imbalance in a semi-conductor (not dissimilar to how a computer chip relies on state shifts to encode binary zeroes and ones).
Anyway, the long and the short of it is that there is no intrinsic advantage in terms of light quality but plenty of downsides to using low voltage systems to power LED lamps. For a start there is significantly less choice as regards the variety of lamps available.
But perhaps the main disadvantage is that sticking with pre-existing low voltage circuits when switching to LED lighting requires not only that the lamps be replaced, but also the transformers and dimmer switches (if applicable). It involves considerably more work and expense and you also need to ensure that the various components are all compatible with each other otherwise it won’t work (properly or at all).
It is frankly a great deal cheaper and simpler to just cut out the transformers and fit mains fittings (typically using GU10 lamp holders). Then all you need to do is push in the LED of your choice. Many mains powered LEDs are also dimmable these days (though you may need to use a dimmer switch capable of running very low loads – many regular dimmers go haywire if the total load drops much below 60 watts). More on the subject of transformers and dimmers here…
Ironically, even mains powered LED lamps, when it comes down to the actual LED chip, use 12 volts. But they incorporate all the necessary electronics to drop the voltage and respond to dimmers within the lamp body, thereby saving you the trouble and simultaneously avoiding compatibility issues.
Do be aware though that even with mains (or “line voltage” to drop in a bit more jargon) LEDs, most are not dimmable (you can easily spot those that are because they will very prominently advertize this fact).
But that’s not to say that low voltage LED lamps have no place, just that they’re a poor choice if a mains voltage circuit is available because they add unnecessary complexity. If however you are constrained to using a low voltage power supply (out in the garden or on a boat or direct solar energy) then clearly in those cases it is the best (on account of being effectively the only) choice.