What factors determine how we decide how to use crops - fuel/food?

Many factors.
1- Is there a food shortage in the area? the answer to this is no. There is plenty of food being produced, just that everyone can't buy it.
2- Is wheat expensive? is it affordable for the population of your area? the answer is Yes, its expensive and yes, its still affordable.
3- Whats priority, fuel or food?

Having said this:
-Use as food: wheat is expensive in other parts of the world, where it has become too expensive for many to afford.
-Whats the point for saving the atmosphere from CO2, when people may starve from this idea.
-Crop fuel is not efficient, and the atmosphere is polluted anyways while harvesting the corn (pesticides, etc)
-Crop fuel is not reliable, as it depends on rain, and plagues, etc.
-Corn is for eating. Fossil fuels are for burning up.

Use as Fuel:
- Saving the atmosphere
- We have to try something new, maybe this is a good answer
- burns cleaner than coal or gas

Overall i completely disagree with using it as fuel, so think of those yourself.