From what I've heard, the Sigma has more rounded bokeh as compared to the AF-S, but have fading(or whatever you call it) towards the edges. It is, however, sharper than the AF-S in the center.
I was shopping for one a few months ago and decided on the AF-D instead. I got a used one for only $260, so that's a big plus. Bokeh isn't as smooth as the other two but I can accept that. AF wise, it's comparable to the AF-S as well, so in general, I don't see a reason to pay more than twice as much for a new AF-S or Sigma over a used AF-D. That's just me. I am a budget shooter.
Note: Bokeh wise, unless you are shooting in a very specific condition, such as object against background with lights, the roundness of the bokeh rarely matters. I do street scene most of the time. Even at night, I find that my backgrounds are always much more complicated looking than plain lights, so having more blades doesn't give me a whole lot.