Without knowing the composition of the particular fuels, it's not possible to give a very precise answer. However, petrol (gasoline) has a much lower freezing point than diesel fuel. The range for petrol seems to be somewhere between -40°C and -60°C, so it's unlikely you will ever have a problem with your fuel freezing if you run your car on petrol.
There are also a lot of different types of diesel fuel, but the "pour point" might fall in a range between -6°C and -18°C (*), although these values can be lowered with additives. The pour point is the temperature at which diesel turns into a sort of jell which can no longer be pumped, so it's essentially useless as an automotive fuel at, or below, those temperatures.
(* Marks Standard Handbook for Mechanical Engineers)