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I use the Sigma 180 and it is a heavy cumbersome bit of tripod only kit with a rather slow and erratic auto focus.
But most macro photographers prefer to manually focus and although moving a heavy tripod in dense undergrowth is a struggle, you do quickly learn a few tips to make it easier.
I think that the decision will probably depend on exactly what you want to photograph. For those nervous species which can't be approached too closely or where there is some limiting physical barrier, a 180 mm does give that little bit more reach.
After using it for 18 months I can't really find any serious problems. But you will need to remove that massive lens hood before doing any flash work.
The Canon 180 is generally rated as being slightly better; but compare the price difference!