The biggest factor, to me personally, is the depth of field. Even when you have plenty of light, the f/2.8 helps to blur backgrounds and make your subject stand out.

Only you can decide how important the weight is to you. Some guys (e.g. videographers) hike all day long with 50 pounds of kit on their back. Others feel that Digital Rebel + 18-55 is too heavy. You just have to decide that for yourself.