gas vs diesel
I am biased towards diesel because of the higher energy content of the fuel, hence longer range; and, simpler systems. Note, I didn''t mention the safety of the fuel, because gas has shown to be a safe fuel source with only a slight increase in safety diligence required.
Gas trawlers are rare and their re-sale value gets hammered. But, if you are going to be a liveaboard in one place and few extended cruises planned, you will be a long time recouping the diesel purchase value increase through the operating fuel economy of a diesel over gas.
Just from an aroma standpoint, turbo diesel power units have made great strides in the last 5 years. The old normally aspirated diesels that were the fare for displacement hulled trawlers are pretty smoky anyways. Should you find your needs justify a re-power to diesel, the new units offer a lot in terms of smokeless and vibration reduction. With a new turbo diesel you would zoom the head of the line in resale IMHO.
Another thing - a 900 hour diesel does not equal a 900 hour gas. Assuming good maintenance and light use, check the exhaust manifold carefully. Even with freshwater cooling, the exhaust manifolds are salt water cooled on the gas units. A pricey little maintenance item around this number of hours/time.