I know people recommend the 105mm for fungi, as the slightly shorter working distance can be more manageable than the 150.
Ask Nick Cantle about the 105, he uses it with truly excellent results for fungi. (and a lot of the time he uses it with a 40D as well
)
I wouldn't hesitate to buy a sig 150 though. The only bad thing I've ever heard said about it is that the AF is (comparatively) a bit slow, but this is common with proper macro lenses anyway!