Ok, first we'll discuss the difference.
Black smoke : The black smoke is usually black because of buildup in the engine and because of the fuel being used. Gasoline and Diesel are both hydrocarbon molecules (ie. they both have a mixture of hydrogen and carbon). When oxygen mixes with the hydrocarbon, and fire (or exreme pressure for diesels) is introduced, the oxygen and hydrogen have a chemical reaction and what's left is CO2 (carbon with two oxygen atoms). Since pure carbon, also called graphite (used in pencils), is black, this would explain why the buildup in the engine and smoke are black.
White smoke : Usually, the white smoke can just be water vapor with a little bit of CO2 in it. But, if it has a light blueish color to it, this could mean that their gasket has a leak and oil is getting burned in the engine.... Not good.
Unfortunately, I cannot answer which one is worse without knowing exactly what they're putting off and how much. But, it is safe to assume that too much CO2 is not good. Since carbon is the closest atomic structure to a black body, its absorbtion and re-radiation of energy leads to a greenhouse effect. This in turn is why global warming scientists are trying to slow the release of CO2, or any other hydrocarbon, into the atmosphere.