Thanks, all, for sharing your knowledge and advice. Ryan, I appreciate your input, particularly since I love your photography. However, the price right now is a clincher, and so is the macro feature. I just don't have the money required to buy a higher-end, or even a medium-priced, Canon lens. In any event, I read your post after I had already pulled the trigger.
Having read good to glowing reviews about the Sigma, including those here by Scott and Chris, as compared to the Tamron's more mixed reviews, I went to Norman Camera this evening and tried out the Sigma. I had already tried the Tamron a couple days ago, so I had good grounds for comparison. The Sigma is clearly the superior lens--crisper image quality, faster focusing, more solid construction, and APO. After snapping a few photos and viewing the results, I was impressed enough that I bought it. The lens is a birthday gift for my lady friend, Lisa, who used to own the ProMaster version of the Tamron and really liked it. Feels good to get her something that's noticeably better to replace it.
Again, thanks to all of you for your input.