Diesel-engined cars are generally noisier, notwithstanding the improvements that have occurred in recent years. At the moment, diesel is cheaper than petrol (it's the other way around in the UK, for some reason).
AFAIK, diesels require more frequent servicing (I'm open to correction though) but are still the most economical option for those who do a lot of driving.
Car tax is determined by the level of emissions from the engine which, even for some petrol-engined cars, can still be relatively low.