If you use a lot of hot water, get a good vacuum thermos so you can boil a lot at once, and then store the leftovers. If you spend most of your time at home, the pump-top ones are convenient, otherwise, you could multi-purpose a travel thermos (less convenient, but it works). Then it will matter less which method of boiling water you actually use. That said, I have an electric. Saves range-top space and uses the heating energy more directly. I didn't do the detailed math for comparison, though.
One not-yet-mentioned advantages of a standalone kettle: doesn't heat up the house in summer.