Thanks for the comments so far guys.
A number of you have deduced the reason I am considering the Sigma is because of the constant f2.8 aperture. I am trying to establish myself as a wedding photographer and I think this would be an ideal lens. My reasoning for this is that I shot a wedding not too long ago with 24mm Nikon and it was sufficiently wide for me, even for group shots. The Tamron I think is too long on the wide end and the DA* is too hit or miss to consider right now.
So it seems down to between Sigma and Tamron.
I would like to ask for users of either lens to post some pictures so perhaps there can be a consensus as to which is the better lens IQ wise. Perhaps there can be some discussion regarding 24mm (36mm) vs 28mm (42mm). In the past I used to use P&S film cameras that were fixed at 36 and 38 and it didn't bother me. But for some reason 42mm does. Maybe I'm losing my marbles. Perhaps some discussion can clear that up,,,,