A diesel engine will go much farther on a gallon of fuel that the standard gasoline engine because of their designs, and due to the higher energy density of a gallon of diesel fuel. But, it also takes a bit more oil to manufacture a gallon of diesel than a gallon of gas, with the production and refining processes for diesel producing more gases that trap heat.
Therefore, when you consider the relative merits of deisel and gas cars, try knocking the MPG estimates for the diesel car down by 20 percent. A diesel vehicle will cost you a bit more, so you'll get more bang for your buck from a gasoline vehicle.
The nasty rumors you hear about diesel are true as well - diesel is less refined than gas, or in other terms it's dirtier. Diesel vehicles also emit more particulate matter and NOx, both of which are serious health hazards and air pollutants. Current diesel engines are more polluting per each mile they are driven than gas engines.
Using biodiesel on the other hand, will improve this situation. If biodiesel is available in your area, you'll still need to examine whether a diesel is the right vehicle for you. When you consider the facts, you have to ask yourself which models you can afford, what is the MPG, will engine be succifient for you, and the number of passengers the vehicle will accommodate. Then, given your budget, you can go from there.