from the digital picture:
"Canon's other current 28mm prime, the Canon EF 28mm f/1.8 USM Lens, is the most logical comparison lens. Even though smaller and less expensive, the Canon EF 28mm f/2.8 Lens nearly keeps up with the f/1.8 in sharpness. The f/1.8 obviously has wider apertures available, but even stopped down to f/2.8, the f/1.8 is not much sharper than the f/2.8 lens. The 28mm f/1.8 is sharper in the corners, but at identical apertures, these lenses are not very dissimilar. What is noticeably different is the corner shading on the Canon EF 28mm f/2.8 Lens. Corner darkening is the most significant differentiator when comparing images taken with these lenses. The f/2.8 has a little more barrel distortion than the f/1.8. Of course the f/1.8 is better built and has USM AF with FTM (Full Time Manual) focusing. I regard the Canon EF 28mm f/1.8 USM Lens, as the better lens, but the better performance and build are not lost on the price."
I love USM, and will never go back. Full time manual focus is a must have, for me. Also USM is fast and quiet.
But if you're on a tight budget, check out the 2.8 and see what you think. You can always sell it if you don't like it or if it's too slow for you.