Diesel, for the reasons mentioned.
Also depends on your tax system. Here, diesel for road use is taxed, and costs us around £0.80 per litre (£3.64 / gal). Diesel for non road use ie plant, tractors that don't use public roads etc.is only £0.35 / litre. There's no such concession for petrol. You have to pay the duty whatever you use the petrol for. You pay around £0.79 / l (and rising) whatever.