Here's a brief summary of my ETTL2 comparison of the three flashes below. I took a bunch more photos but they showed, as the photos below, that there is no discernible difference between the way these flashes handle the same lighting situation. I tested direct flash, and bounce from a different angle and used a couple of different distances and subjects. The results were all very consistent. I used a Rebel XT and A Rebel XTI in the test.
Also I regularly use the Sigma and the Quantaray, and they have different strengths and weaknesses. The Quantaray has a audible squeal, occasionally seems to have a blonde moment (not firing, or delayed fire--some user error may be the cause, which I suspect) but is easier to change settings quickly. The Sigma has more settings and features, is silent, but requires more button pushing both for flash settings and for rotating and angling the flash head.