I have the Tamron Canon version.... And it's more than an able Lens for wildlife....
I think some of the deciding factor has to be weight... The sigma is a kilo heavier than the Tamron, and the Tamron is 2kg in weight....
And believe me, lugging a combination total of 4kg's (Lens, 7dmk2 and monopod) stalking Red Stags on Exmoor during rutting season for 8 hours was totally knackering and took some doing....
The sigma probably slightly superior on focusing speed, so theory wise should hit focus more often, but saying that it more depends on the camera capabilities, if the camera slow to focus, then you're not going to gain much advantage with a fast focusing lens!
I just teamed my Tamron up with a monopod and gimbal head, and even though freehand I've got some smashing shots, was still surprised how much difference it made against hand held... But again tis heavy and I'm not sure that the sigma slightly superior edge would sway me, due to lugging another kilo of weight around..