I'll try to give you my opinion, since I'm also interested in this lens, presently.
Sigma 180 f3.5 is a very good lens for macro.
However, a tripod is recommended for the use of this lens, becuase of his weight.
For use of lens on hand, maybe the "100" is a better and confortable choice.
At the moment, very recently, I bought a Canon EF 100 macro lens, postponing in the next future the purchase of 180 macro (because it allow you to stand far form the subject and, specially, the blurred background is more effective and nice for picture).
For 180 lens I'm oriented to SIGMA most probably, even if someone recommeds TAMRON; the reason: SIGMA is affected by poor quality control system, that means: some copy are very good, some else are defective; TAMRON has a uniform level of quality on all copies.
I hope this note can help your decision. Ciao.
_______________________________________________________________________________________
La grande foto è l'immagine di un'idea
(Tiziano Terzani)