I own the Sigma 105. I originally had it on my D90 before getting the D300s. I absolutely love it. Youll adore the extra reach the 105 gives you, even if it is extremely minimal. Everyday I wish I had the 150 or 180mm. The Tamron is renowned for being a wonderful lens, especially the 90mm. I, personally, think it is a tad short for animate critters such as bugs. It is known for being one of the sharpest macro lenses, however. Almost all macro lenses from all four major companies are sharp..its just the nature of macro lenses, but the Tamron 90 is known for being among the sharpest, as much as possible. Ive never had sharpness issues with my Sigma, however. I would save for the Sigma 150 or 180, personally. If you want something shorter, Id go for the sigma 105 because of the little bit of extra reach given. Its a wonderful lens.
Mark