Both the Tamron and the Sigma that you link in the above post are similar in features.
The aperture is 2.8 on these lenses. That means that the widest the lens opens is 2.8. The smaller the number, the wider the lens opens to let in light. A fast lens is one with a large aperture. If you are taking pictures indoors or in low light, you generally want a faster lens.
The Sigma 17-70 isn't a constant 2.8 across it's entire range. Meaning at some focal points the widest it will open is 4. However this lens has further reach (ie. 70 versus 50).