I do have a lot of Nikon lenses which generally compare well to Canon.
I don't know that a new lens is going to help you much with food photography. Lenses tend to perform best in the middle range of their aperture extents. Most lenses will perform their best as far as resolving power goes when stopped down one or two stops from the maximum aperture. With food photography, the subject generally doesn't move and you can put your camera on a tripod to keep it from moving. As such you can select any aperture setting you like in just about any lighting condition. Even kit lenses will generally perform very well at their optimum aperture setting. For food photography, if I had to choose between the 18-55 and the 50/1.8, I would probably pick the 18-55 because it focuses much closer and covers more focal lengths.
If you want to improve your food photography, the best way IMO is to focus more on lighting. Food photography is all about the lighting. You don't even have to invest that much, if anything. Learning how to strategically place desk lamps and modifying that light source in different ways with cardboard is a great way to do it. Building your own light box is also fun and cheap to do and will improve food photography immensely.
http://strobist.blogspot.com/2009/06/food-photography-made-easy-lunch-box.html
If you want to buy a fast lens for other types of photography, a fast lens in the normal to short telephoto range is a great thing to have. They don't take up much room or weight in the camera bag, and there are all sorts of situations where they come in handy.
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