Are there any disadvantages of the Sigma 150mm macro compared to the Nikon 105mm macro?

They both have image stabilisation and both have a 1:1 magnification, but the minimum focusing distance is about 8 cm longer for the sigma lens. I'm wanting a macro lens that gives a good amount of distance between the subject and the lens, however the sales person told me that they think that the sigma 150mm is too long for a macro lens and thinks the 105mm is much better. Apart from the sigma being slightly heavier, I can't see what the disadvantage is in having a longer macro lens? Does anyone know?

Also does anyone know if the Kenko extension tubes made for nikon lenses will work with a nikon camera with a sigma lens, and keep all of the autofocus and aperture adjustments when using a sigma lens?

Why do you need to shoot macros at more than a 1:1 image to subject ratio? (referencing your question about using a generic extension tube. To know if the extension tube will retain auto-focus and metering with the Sigma, you will have to contact Sigma

If you are coping flat art or documents using a copy stand, both are too long. You will need a lens like the 60 mm f/2.8

The 150 gives you a really shallow DoF. From what I hear it's a challenge to shoot handheld.