Some things have changed recently that have combined to reduce diesel production, which naturally puts the price up. There's the transfer of some refineries to petrol production, the move to ULSD (ultra-low sulphur) and the closure of some diesel refineries (I don't know the details of that).
ULSD has helped a lot, but diesel still contains toxic nanoparticles and fine soot. I'm not aware of any direct comparisons, probably because of the different substances and carcinogens in each type of fuel, but I'm sure that info is available somewhere.
Diesel is more economical because of its density (about 15% more) and its energy content (about 11% more).