Electric water heaters are generally much more energy efficient, since a very high percentage of the energy used goes directly into the water temp. Conventional gas water heaters are much less efficient. BUT in many areas, due to RATES...gas is overall less $$$$ to operate.
We have been living with natural gas in our houses for I guess over 100 years. Just like wood, or coal, or electric,....there is the potential for accidents, but with proper respect, I don't think you can make a case that gas is more likely to cause a fire than an electric.