So whats the difference?

I suppose the main difference is that a low voltage system consists of what is known as a selv. This provides a double isolated supply method to the lights at a non fatal voltage. The transformer provides earth isolation to reduce the risk of electric shock, although this varies from one product to another. The down side is that you may suffer from voltage drops in a LV system. In a 230Vac system the voltage drops form a lesser percentage (considering constant impedance and ohms law against I squared R losses) and thus you can kind of ignore them.

Mains halogen downlighter bulbs: shorter lived lower efficiency, ie more energy hungry more fragile bulbs more expensive and blowing bulbs are liable to trip your lighting circuit.

LV halogen has none of these problems, the =A36 for the transformer is well worth it.

NT