I was once so adamant about just using a gas flame to do my cooking that if I was in a kitchen that only had an electric stove, I would buy a small gas camp stove to cook on. However, with all my travels and cooking in so many different kitchens, I've mellowed out a bit and realized that the love and good energy I put into my dishes can alleviate any negative energy it may pick up, so I will use whatever heat source is available. I got to cook on a solar oven a couple of times, and even though I love the idea of using natural heat sources, it's not really practical to have to wait till the sun's at the right angle to boil my water!
My preference is a gas stove for the obvious reason that a flame is so much easier to control and it is a more natural element to work with. Whatever stove you're using, in feng shui, it's recommended to rotate the burners you use as the stove represents prosperity and this will ensure your money supply will come from various sources—I'm down with that!