Pollution output is better, theres no argueing that. Just remember back to the days of the Oldsmobile diesel engine. Even the large displacement off-highway diesels are cleaner. Cat has replaced the 3500B series engine with the C series. Same engine, same hp #s, but using bigger turbos and fuel settings the engine burns even cleaner that the B.
Natural Gas engines are getting cleaner also. New NOX systems are replacing the O2 systems. I personally like the new systems, they make tuning and performance troubleshooting alot easier.