a flat iron straightens your hair Paula. For those of us with curly/wavy hair it gives us the best of both worlds. The ability to choose when you want it straight or not.
If you have hair that is just a bit more then just waves it will straighten it. If your hair is REALLY curly it would be best to blow dry straight first then flat iron. The flat iron will usually have something in it that takes out the static so it is good to follow even the blow dryer with a flat iron so it will lay more flat. They are both damaging. I really don't think there is a lesser here.