Here are three possible candidates for “standard” zoom:
On the face of Sharpness the Tamron looks good: 10P-Mpix is just higher than the Sigma.
Both the independent lenses beat the Canon which scores just 7P-Mpix.
Tamron lens does do very well, but it is far less consistant: at its best it just beats the other two but wide open it has to be at 50mm to win and stopped down it really falls apart.
The Sigma on the other hand maintains a similar result throughout the range of focal lengths and does better at retaining quality stopped down.
The DxOMark choice:
The first point is that the Canon is not as good. It’s slower by a full stop over the Sigma and it lacks the sharpness of the other two.
The Tamron will probably produce the nicest results at its optimum aperture and focal length but the Sigma will come close and just beats the Tamron overall.
Additionally the Sigma is cheaper, lighter by 100g, zooms out to 70mm instead of 50mm and offers a greater degree of magnification for Macro: 1:2.9 against the Tamron’s 1:4.8.
It is good to see when companies introduce an updated version of an existing product and it shows a real improvement. Sigma has had a 17-70mm macro zoom lens in its range since 2006. Initially it had no image stabilisation and the AF was not Hyper-Sonic, but physically it looked very like the new version. Version two had the IS and HSM but was quite a bit heavier. The progress in quality has been good, P-Mpix has risen by one point each time and the overall score has climbed two point each time. The original version had less distortion than the subsequent models but the distortion is really not a serious issue with this lens anyway.
A final brief comparison with the Flagship ‘standard’ APS-C zoom lenses from Nikon and Canon, admittedly these are really marketed to a different sector but the figures are quite sobering. Both companies produce f2.8 lenses with a 17-55mm range, both have a price tag twice that of the Sigma, both are much heavier and larger (quite understandably given their faster aperture). However the Sigma from a quality point of view is as good as the Canon and better than the Nikkor. If it is not essential to have the fixed f2.8 aperture then the Sigma lens is really the best value here.