Its better to make it from plants, if the economics around it aren't predatory. From plants, its carbon neutral - I don't think we're at a problem point with carbon dioxide, but it will surely come eventually. Plants get their carbon from the air, so if you burn a plant as fuel and it goes into the air, the net effect is nothing. But the economics around it matter a lot. I mean, who grows it, or is allowed to grow it, and what their inputs are. Fertilizer based agriculture is the thing destroying the planet. It has to stop. Petrol is not the main problem, not even close.