I know Prebennorholm has a better answer for comparing the flammability of the two fuels, but I'm inclined to guess that gasoline is more explosive just because it seems like its more volatile than kerosene, so you've got a better chance of rapid combustion of vapors.
Cars don't usually burst into flames in a crash because, unlike planes, they don't generally go belching the contents of their fuel tanks on to a hot engine and into the passenger compartment. Of course engineers for both kinds of vehicles try to prevent this from happening, but it seems like the aircraft designer has many more constraints on implementing safety measures than an automotive designer (weight, balance and capacity to name a few).
Just out of curiosity, how many car and plane crashes (non-Hollywood) have you seen? (Personally, I always liked it when the bad guys' car blew up going over the cliff and the Duke boys got away again.)