Diesel is more energy dense than gasoline but it's traditionally quite a bit more expensive per gallon. Lately diesel has been just a bit more expensive than regular gasoline so at the moment diesel is the winner.
To make an intelligent decision you'd have to compare the MPG of the available gas and diesel engines and the cost per gallon of fuel to see which fuel gives you the best return for your money.
You also have to look at the useful lifetimes of gas and diesel engines, the cost of normal maintenance and the cost and availability of repair parts.