These 2 lenses have different focal length, it depends on whether you want to have a wider angle view and what type of body you are using it with. The Sigma is only suitable for APS-C size sensor while you can use the Tamron for full frame camera. But the Sigma allows you to go wider on APS-C size camera.
I bought the Tamron 28-75mm f/2.8, for the constant 2.8 aperture and the ability to use with my film body. I have another ultra wide zoom to cover the wide angle view. You may prefer to have Sigma for the larger range of focal length. Both lenses are quite good for their price.