Rude has nothing to do with it. This is a BUSINESS transaction, nothing else. There is no difference in what you are doing here and car shopping. You have found something more compatible and more for you. You have not given any money, a horse has not been moved. They shouldn't be counting chickens before they hatch. If they need an explanation, give them one. You found a better horse, simple as that. If you feel you need to apologize then do so, but I don't think you are being rude at all. Go for the better horse.