Watts are watts are watts, so a 12V 50W bulb is going to use exactly the same amount of power at as a 24V 50W bulb, a 120V 50W, or a 240V 50W bulb. Better decide whether you like your crow rare or well-done, I'm afraid.
Whether the bulbs are more efficient (more luimens per watt) is another matter, but if they're the same wattage, they'll consume the same amount of power (with the 12V bulb drawing 20 times as much current as the 240V one). The losses in the xfmr have already been mentioned.
I remember a job two or three years ago where the homeowner had bought two sets of dichros to replace a couple of pendant lights in a hallway. They were those supposedly "trendy" types with three spots on a bar shaped like a lazy S. ("Bar lazy S" -- Sounds like we've strayed into cattle country!
The owner was convinced that because they were low-voltage spots ("and so neat and small too") that they would cost much less to run than the existing lights. I had to explain that with three 50W bulbs on each bar, there would be a total of 300 watts running (not to mention xfmr losses). The kicker was that the two existing pendants had already been fitted with 11 or 16W compact fluoros.
[This message has been edited by pauluk (edited 01-29-2006).]