I've been doing emission test cycles for both modern diesel and gasoline cars. The emission levels of both of them are very low ,and barely measurable with a ten year old equipment. There might be slight advantages for gasoline engines when it comes to certain pollution components, but the most inportant factor IMO is the better efficiency of the diesel which therefore has lower CO2 emissions.
When it comes other pollution components than CO2, I think both engine types have reached a level where the enviromental effect is a small issue. Of course development is not a bad thing, but the most important thing now is to reduce fuel consumption, and that of corse reduces all emissions.