Try stopping down a little. The 50/1.8 sharpens considerably past f/2.8 (as you would see if you just played with the lens comparison thingie I linked to).
With that comparison thingie--the cooler the colors are, the sharper the lens is at that area. It's basically an MTF chart over the entire aperture range. The f/1.4 and f/1.8 lenses are laid out side by side for you. You spin the wheels on the aperture to see how the lenses perform across the aperture range. The f/1.4 is on the left, the f/1.8 is on the right; both tested on a crop-body sensor (you can switch to a full-frame sensor if you want). The farther apart the red and blue lines are, the more chromatic aberration there is. If you look, the sharpness and chromatic aberration are pretty close across the range, with a slight advantage to the f/1.4. So, if you want a ton more sharpness, I wouldn't count on getting that by going to the 50 f/1.4.
Autofocus performance, however, is probably faster with the 50 f/1.4, because it's USM, and 1/3 of a stop brighter wide open. No idea if it's more accurate, though.
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