For simple tasks such as reheating, preheating, boiling water, and defrosting using the microwave verses the stove is the more energy efficient choice. Microwaves work faster and more energy is aimed directly at the food as opposed to the container and the area around it.
The microwave (most modern options) uses up to 75 percent less energy. Along with using more energy the stove will also put off heat, increasing the temperature in your kitchen. This may not be bad in the winter, but more heat is the last thing you want to add to your kitchen on those hot summer days.
There is a lot of talk over microwaves and whether they are the greener option. I would have to do some more research before I debated that issue. I'm just saying when it comes to energy efficiency, it will typically cost you less to use your microwave when possible, instead of the stove.