I've got the Sigma and am very happy with it, as are most owners. The Tamron was a consideration but I didn't see enough to convince me the premium was warranted for my use.
Fair points, like I said just my opinion but I have used the Tamron predecessor and Sigma 105 and found the sigma better. Whilst no longer comparing like with like, I have used the Sigma 150 f2.8 OS and that beat any Macro lens I have ever used and could be used for action shots too. The OS was only ever needed for action shots. Macro always done manual focus with OS off. things may have moved on but the only Tamron lens I had that I felt delivered was my ADII 80-210.
Has it occured to you about water drop photography that involves water? Or many other macro subjects involving liquid? It is also a stunning portrait lens, which means you could be carrying it out and about and get caught out by a shower, or sea splash perhaps It also mean you could take it out in the snow for some nice macro shots The weather sealing is worth its weight in gold 25% less weight makes it easier handling too for those macro shots ;)
Never occurred to me to take macro pictures when It is raining. I don't think what its with 50% more on the price personally. Just my opinion.
Because the Tamron is weather sealed, lighter, scores a higher DXO mark, smaller aperture, 5 years warranty