Have you considered the Canon 100mm f/2.8 Macro? That is a loved lens, one I've been eyeing, but I'm not sure how important the focal length difference is, not really sure what you're aiming for as you've given no background. The 180mm will give you the distance, which would theoretically be good for experimental work. But 100mm will give you a better distance from your subject for following things like insects. So if you didn't know that already, possibly consider the similarly priced, and raved about Canon 100mm macro? Check it out if it's of interest. Canon, in my opinion, is the best photography lens maker, and would last you far past the worth of it. Too bad they don't offer a lower end option in that focal/price range.
If you did know that, and you are set, I can't give personal reassurances on either of those lenses, but everyone at FredMiranda seems to think highly of them. Go in and see specifics, but I didn't see anyone complaining about the autofocus, at a glance. http://www.fredmiranda.com/reviews/
And the Sigma is looking to have no complaints.
I'm really curious as to why they would discontinue a lens on one body but not another. Don't know how they work, but I'm betting there was some specific issue with Nikons only or something.