I'm still new to photography, I've done a great deal of research on sigma,nikon, tamron etc... There are a lot of online photo forums where you can read up on each lens, helped me considerably in making my decisions.
Lens are a huge investment, always try and buy the best glass you can afford as there is nothing worse than buying something and then a couple of months later deciding that you should have gotten the "Other" glass.
What you'll find is that each manufacturer has a certain lens that has gotten praise for it's optics vs the competitions same lens. I'd do some online research on each lens and then start comparing. Most retailers will have demo glass, doesn't cost anything to walk into a store and try a lens out.
If your considering the Sigma, go with the 105 EX DG (all new sigma macros are EX) which you can find for around $400 on most of the trusted sites. Most reviews have been positive with respect to the optics, only negative is that it's slow in autofocus, but you should be in manual when doing macro.
I'll post before you next time.
