Pavel Toman , Mar 03, 2004; 12:20 a.m.
Personally, I wouldn't use word "superior" when comparing these two lenses. Both of them belong at the very bottom in the same cattegory.
Anyway, I would probably buy Tamron for its better built quality. And, some may think otherwise.
"I've also heard that both of these lenses are comparable to, or perhaps even better than, comparable offerings from Canon"
Canon, at the very same bottom cattegory is just as crappy. The built-in quality is better then that of those mentioned above, though. Optically? Not much of a difference, but surely not worse. The USM version beats both Sigma and Tamron and IS USM takes it to a different league.
Macro capability that both sigma and tamron have to offer is a nice thing for a beginer but useles for more serious work.
The only good thing about those lenses from my point is their versatility - compact, lightweight, incospicuous, cheap, let you reach out where point-and-shoot cameras won't let you.
#1. canon 70 - 300mm IS USM
#2. tamron ....DL
#3. sigma ....APO