Hi Toby,
I have the Sigma 150 but have never used the Tamron. Having said that I used to use my Panasonic FZ30 with close-up lenses for macro, so I know all about having to get close to your subject!
The Sigma is a very nice lens, but is very difficult to hand hold. I have pretty much given up handholding because it just doesn't get the best from the lens. From what I've read, I don't think image quality will be noticeably different to the Tamron, so the only gain you'll get is the greater working distance. This is very useful and is something I appreciate after using my FZ30. However, I don't think the Sigma 150 offers
that much extra working distance than the Tamron and the chances are if an insect will let you get within 3 feet of it then it will also let you get within 2 feet of it (or whatever the measurements are). I typically find that an insect either won't let you within 6 feet of it or it will let you get within inches of it.
The other factor to consider is your extension tubes. The extra magnification they provide is measured by the extension tubes in mm divided by the focal length of the lens. So on a 'long' 150mm lens they have less impact than on a 90mm lens. I tend to use 68mm of extension tubes which gives me an increase of 68/150 = 0.45. So I can achieve 1.45 : 1 magnification (if my maths is correct). To get an equivalent increase in magnification on your lens you would only need 40mm of tubes and so lose less light. Alternatively, 68mm of tubes on your lens would give you 1.75 : 1 magnification.
So, yes the 150 lens is a very nice lens but I'm not sure I'd give up what is already a very good macro lens in order to get it. There are certain things that your lens is better at (handholding and extension tubes). So I think you need both lenses
Matt