My first question would be, why would you prefer
a 150/180mm macro lens over a 100mm macro lens?
Is this for the distance to the subject?
In that case, let us have a look on the effective focal length that these lenses
have at MFD (minimum focus distance).
The Sigma has MFD of 38cm and has a focal length of 95mm there.
The Tamron has MFD of 47cm and has a focal length of 118mm there.
The Canon 100L has MFD of 30cm and has a focal length of focal length of 75mm there.
So, regarding the focal length I would prefer the Tamron over the Sigma.
On the other hand, if you look especially for the support of in-lens image stabilisation,
I would really have a look on the EF 100L (especially, since the price for the Sigma is
quite bold as well).
In all, I believe that with macro-lenses you don't really make a mistake regardless
which one you take. They are all quite good.
Just my 2cts ... Rainer
1. Sigma 150 OS is not available. We all hope it will be (along the SD1) before the year's end....
2. My assumption that the Sigma will be the more versatile lens due to faster aperture and stabilization but the Tamron will still have an edge WRT insect photography as it has 5cm greater WD.
3. A personal note: For a long time I've waited for the 150/2.8 OS but I got frustrated by the time it takes them to release it so I got the 100/2.8 IS. As I don't shoot a lot of insects it's actually a better lens as it's smaller and lighter.
4. What is your current gear? Why is it not satisfactory? What will you be using the lens for?
Welcome to PZ.