Call it the battle of the environmental advocates. Vinod Khosla, the esteemed entrepreneur and venture capitalist, held forth on his plans to replace gasoline with ethanol during the first of two tutorials on the environment Wednesday afternoon. Lester Brown, president of the Earth Policy Institute and author of the recent Plan B 2.0: Rescuing a Planet Under Stress and a Civilization in Trouble, presented his competing view right after Khosla. What we should have done is brought the two together for a green slugfest.

Khosla has become perhaps the best known proponent for ethanol in America. His slideshow, less multimedia than Al Gore's but just as powerful, explains why ethanol can end our addiction to oil. (Khosla was featured prominently in Fortune's January article on ethanol, "How to Beat the High Price of Gasoline. Forever.") Khosla's policy proposals include mandating that 70% of all new cars be able to burn fuel that is 85% ethanol. He's heard all the criticism of ethanol before. To the people who say ethanol will use up all of America's crop land he says that they are assuming no advances in crop technology, which is wrong. Dramatic crop yield improvement is a given, he says. He also notes that the future is not corn ethanol, largely used today as a gasoline additive, but rather cellulosic ethanol, which is made from the garbage of the agriculture world. Khosla thinks his dreams could be realized in a matter of years, if government and industry (minus the oil companies) pull together.

Brown has a different take, to say the least. He sees ethanol production as being incredibly wasteful. Fill up your SUV with ethanol, he says, and you're doing nothing for the environment. Moreover, Brown points out that growing ethanol, any kind of ethanol, requires water and that half the world already is pumping more water than they should be. Countries like the U.S. and China are depleting aquifers so rapidly that there won't be enough to grow food, let alone renewable sources of energy. To be sure, he thinks cellulosic ethanol will be part of the solution. But he worries about the rise of corn-based ethanol creating a competition between fuel and food. He advocates gas-electric hybrids powered by wind-generated electricity, and a carbon tax offset by income tax reductions (now being considered in China and Japan.)

We were so impressed by the differences between these two thoughtful people that we're trying to set up a debate between them Thursday. Stay tuned.