An aperture wider than 2.8 isn't the optimal aperture for sharpness on either lens. The copy of the sigma 17-50 I had owned was quite sharp at 50mm and I had found it comparable to the sigma 50/1.4 I had at the time at 2.8, which was no slouch either. I know its not quite the same lens, but check out the in-depth 50mm comparison
here to get a general ideal of what to expect between a 1.4 and 1.8 lens, the differences are there but you really have to try and look for them as they are quite negligible.