I have both of these lenses and both take excellent pictures. I actually like the Sigma better because of the following: I like the extra 15mm in focal length and the extra 2 or 3 inches of working distance when doing macro.
The Tamron has a slightly smoother bokeh, but the Sigma gives more color definition in the OOF portions of the picture, and I really like that. That, however, has a lot to do with personal taste. I agree with the post above. The Tamron IS slightly easier to use because of the one step switch from macro to AF. Some more considerations: The Tamron is warmer than the Sigma, and the Sigma a little darker than the Tamron, but those things can be compensated for with camera settings or PP. I posted pictures comparing the 2 lenses in another thread, but will do so agan here. Please don't judge the sharpness of these pictures because they were all done hand held in manual focus.

Sigma. Notice the colors in the leaf. (sorry that the focus is a tad off)
http://keemra.smugmug.com/photos/49220743-M.jpg

Tamron. Notice the warmth and smooth bokeh
http://keemra.smugmug.com/photos/49220740-M.jpg

Another from Sigma.
http://keemra.smugmug.com/photos/49219856-M.jpg

The same shot from Tamron. Notice how the distant part of the leaf bleeds into the background and becomes invisible.
http://keemra.smugmug.com/photos/48943482-M.jpg

All 4 of the above pictures were taken wide open. Two more pictures and the EXIF for these ones are in this thread if you are interested:
http://forums.dpreview.com/forums/readflat.asp?forum=1030&thread=16376350
If you have any more questions, please feel free to ask.
--
Joanna
http://keemra.smugmug.com