Actually, I'm more concerned with the working distance from the subject for these two lenses. Does the Sigma in macro mode at 300mm means I can take the subject further away compared to the Tamron 90mm? Thanks.
90mm is true macro, 70-300mm is not, besides, 70-300 only can take macro from 200-300mm range.
300mm definitely can take from further, but not 1:1, as mentioned earlier. I think 1:2. With extension tubes/teleconverter no problem though; but people have commented that it is not the sharpest of lenses. But it is very good value for money, and I will definitely get it sometime in the near future if I have the cash.
I find 70-300 very good for taking butterflies. Most of the butterfly chasers here advocate the 180mm macro, but the 300 range is unbeatable, very safe. As for smaller insects you'd probably want a true macro lens.