Diesel particulate matter is extremely dangerous because of its size and ability to block lung alveoli. (PM0.1-google it!) However, I don't know if "clean diesel" is that worse or better than gasoline cars in terms of PM. I know the sulfur content of the fuel is just as important as the car itself. Personally I like CNG as its cheap, burns the cleanest.
On a pure efficiency basis its obviously better than non-hybrid gasoline vehicles.