The Digital Picture has an extensive database of test charts shot with virtually all Canon lenses under identical conditions. Here's a comparison of the two you're looking at on a full-frame camera at f/2.8:
http://www.the-digital-picture.com/R...mp=0&APIComp=2
You'll notice that below f/2.8 both have nasty aberrations even with black lines on white background - stars will be horrible.
The two lenses are worlds apart in build quality, focusing speed, manual focusability, and bokeh (background blur - very harsh on the f/1.8). The f/1.8 version is popular primarily because it's cheap and provides great value - image quality and f-ratio that normally costs many, many times its price.