Macro lens choices?

I am new to macro photogaphy. I have an 80/400vr Nikon for wildlife and a 12-24 Nikon for wide landscape shots. I am also pretty sure that I am upgrading my body to a new Nikon D7000 soon.
I am looking to start to get into macro but don't want to blow the bank on a lens for it. I have seen two used lenses that interest me and are at the max range of my budget. They are: Sigma 180mm f/3.5 EX DG APO Macro IF HSM Autofocus Lens for Nikon AF and the Nikon Telephoto AF Micro Nikkor 105mm f/2.8D Autofocus Lens. The Sigma is rated a 9+ and the Nikon is rated an 8+. Which would you buy or do you have a better suggestion? I figure we are talking bugs, flowers and butterflies to start with.

IMO, the Nikon is the best by far - and will also hold it's value much better.
All my flower and macro shots are with the 105, if you want to take a look at my gallery.
If you don't want new prices, check out KEH.com and Adorama - both have great and honestly-evaluated used lenses.
Sandy

I've used the same Sigma 150mm f/2.8 Macro lens for over six years.
I believe it's the best/brightest macro lens out there, the optics can't be beat.
It has seen a lot of use, still works as good as the day I bought it.
I highly recommend it

The Tamron line of lenses are excellent and won't cost you your first born child. I have both the 90 mm and 10 mm and they have not failed to yield quality images.

I also have the Sigma 150/2.8 Macro - said by some to be the sharpest Macro out there. It's autofocus, however, completely sucks, which is not really an issue for Macro, since you should be manually focusing - but it's really irritating to try and use as a telephoto. It is quite sharp, though, and I'm very happy with it.

I have the Nikon 105mm f/2.8 VR and love it. The VR is great, and one added bonus to getting the Nikon is it is compatable with the 1.4, 1.7 and 2X TC's. I think you would be happy with either the Sigma or Nikon and would be hard pressed to tell a difference, but little things like TC compatability may sway you.

A new Sigma 150/2.8 is coming out with OS sometime in the near future. I've seen the specs on Sigma's site, but I can't find any information about a release date. The OS function should make a huge difference for handheld shots in natural light.

A review of the new Sigma can be found at http://www.dpreview.com/news/1009/10092118sigma150os.asp

I'd go for the longest lens you can get - working distance and subject isolation are key for macro photography of live bugs (in my opinion), and the longer Sigma delivers that for sure. If you buy used, make sure the centering quality is decent.

I use two lenses for macro photography myself - one is the 105/2.8 AF-S VR Micro Nikkor (I use it for flowers, as a general telephoto, and for "informal" macro shots, where I don't have the time or space to set up the real thing - it's an excellent lens), and the other is a 400/5.6 prime lens to which I mount up to 70 mm of extension tubes (I use this for large working distance and excellent subject isolation).

The used Sigma lens is a 180mm 3.5 NOT a 2.8 that is one reason why I am asking the question.

ok I went a little crazy and bought the Sigma 150mm 2.8 EX APO HSM that a lot of people raved about. None found used so I broke down and got a new one. Don't tell the wife how expensive my upcoming bug and flower pics are!

Talk about Mr. Undecided! I just cancelled the order because I see that Sigma is releasing the SAME lens with the OS feature in about a month or two. I called them and they said that the new lens won't be much different in price and the rep even told me that they thought it would be worth the wait and don't get the older version without the OS.