The first thing you should do is sort out in your mind what kind of photographs you will be making, and therefore which range of focal length in a zoom lens would best suit your purposes. Once you know that, you know which lenses are available. If there is an L and a non-L having roughly the same focal length range that suits you, the next decision is how badly you need the build quality of an L lens. Putting it on a Digital Rebel, I'd wonder whether it is over-kill. The next question to ask yourself is about the extent you will actually see and need the quality difference between an L and non-L lens. To do this, buy it from a dealer who will let you test it and exchange it if you want something else instead. Under these conditions, I would start non-L if there is one in the focal range you want, and see whether the images are satisfactory for the biggest enlargements you will make. If so, your done. If not, try an L. Bottom line: you need to go through your own process of elimination to select what is best for you. The foregoing is a guide for doing that.
