It's best to have both. It only cost about 200 dollars a year. In "82" a hurricane totaled my mobile home. The insurance company didn't want to pay because of flood damage. I had a neighbor that witnessed the roof opening and after that they paid. I accidentally let it lapse a couple of months ago and was concerned. I'm not in a flood zone but if you have both then you don't have to worry about technicalities. The only thing that will help the Katrina victims now is some kind of structure damage that let the water in.
When I first bought flood insurance, 11 years ago, it was $200, or so. The last time I bought it, it was $700+.