Low voltage lighting

I am looking at changing the lighting in a room in my house. The wife wants downlighters so I have the choice of using mains voltage or low voltage through a transformer. My local DIY shop sells a set for £15 which contains a transformer, wiring loom and the three halogen dichroic lights of 20W each. Now if I were to fit 3 mains voltage 20W lights (which incidentally cost more to buy) would they use the same amount of electricity as the low voltage ones bearing in mind they are both 20W per bulb or are the low voltage bulbs equivalent to a light output from a mains voltage 20W bulb but actually a lesser wattage?