I have ran both Deere mowers with Yanmar Diesels & Kubotas with Kubota Diesels and both engines were very smooth runing. However when at idle the Diesel engines will rattle a little bit but at full power they are very smooth running and fuel efficant. The new Diesels are much cleaner then Diesels in the past because of tight emission regulations, which I am sure is to continue in the future.
Gas mowers are smooth running as well, but I would never go back to a Gas engine that is carburated,, If I were to go to another gas mower, it would be Electronic Fuel injected mower, much easier to start then a gas mower that was carburated.
Diesels are also more fuel efficant as well and Diesel prices are cheaper then gas ( at least in my area) and at $1.09 Canadian a litre for gasoline you can go through a lot of gas in a rider. I have since switched to Diesel mowers and on average I am filling up at 0.95-97 cent's a litre for Diesel, plus I am not going through as much fuel either vs gas. Then again I am cutting grass commerically as well, but if your just doing your own lawn, a gas mower would be more economical, unless you got a big property and can justify the Diesel.
Take Care Jason B