The Sigma did a decent job for its age, with less distortion at the edges than I had expected. It tended to overexpose images but that can be accounted for much of the time. It did not come with a hood, and its loss had a major effect on contrast; veiling flare is a large deal with such a large element! I could hold my hand out to shade the nose and contrast was much more acceptable. I tried near and far shots and felt the images were quite good, so only the flare was a big deal.
Then came the Tamron. Despite its range it could not match the Sigma for close focus, but at 40mm the max magnification was fairly close. Images were very nice and well saturated, exposure was nearly spot-on compared to the prime lens, and I had a hard time getting much flare or compound reflections in the shot. It is definitely an SP version with superior coatings, even though it's about the same age as the Sigma.
So while I'd prefer to keep the prime for its size and closeup value, the Tamron is sufficiently better in the shots I took (which were reasonable representatives of how I use a lens).
If the next owner finds a hood for it, the Sigma will serve them very well!
