In response to a thread in the Sony forums, here are some comparisons between the Sigma 24-70 and the Tamron 28-75 in terms of bokeh.
#1) Tamron 28-75mm @75mm f2.8
#2) Sigma 24-70mm @ 70mm f2.8
For all intents and purposes, you can't tell the photos apart. The bokeh in both the Sigma and Tamron lens is not too obvious or distracting.
#3) Tamron 28-75mm @ 75mm f2.8
#4) Sigma 24-70mm @ 70mm f2.8
One of the original issues raised was regarding the circular bokeh that results from out of focus points of light. I'm no expert but this should
be present in all lenses. Shots #3 and #4 demonstrate this. The only thing you could say is that the discs are brighter in the Sigma photo, so that might be classed as more distracting.