Comparing two very similar lenses like the Sigma 18-50mm f/2.8 EX DC Macro and the Tamron 17-50mm f2.8 SP XR Di II LD VC Lens in terms of sharpness can be difficult.
Often one lens will perform better at a particular focal length and aperture, while the other will perform better at other focal length and aperture combinations.
Even at the same focal length and aperture, one may be sharper at the center but not as sharp at the edges as the other lens.
When two lenses this close are considered, copy-to-copy variation for either lens may be more significant than representative examples of both models compared to each other.
As other answers have stated, there doesn't seem to be many places that have reviewed both of these exact models, so even comparative test charts like this one at The-Digital-Picture that uses the earlier non-Macro version of the Sigma aren't definitive. The non-Macro Sigma, by the way seems to be sharper overall than the Tamron, but not by much when the different magnification sizes of the charts are taken into account. (The non-VC Tamron, on the other hand, is clearly sharper than either.)
I did find a straight up comparison at DxO Mark. When I first selected each lens, the default camera body was the 7D for both. I was then able to select each lens as tested on a Canon 50D.
At the widest aperture of f/2.8 both lenses are very similar at the widest focal lengths. From 35mm to 50mm the Tamron is slightly sharper in the center but not quite as sharp on the edges. Center sharpness is very close for both at f/5.6 across the focal length range. At the edges at f/5.6 the Tamron is barely sharper at the widest focal lengths but is progressively sharper on the edges from 35mm to 50mm. At f/11 it is pretty much a dead heat from 17/18mm to 50mm. The Tamron measures an almost undetectable amount sharper at 24mm and 35mm.
Overall, the Tamron tested a little sharper than the Sigma on the 50D. In the real world I think copy to copy variation could well be greater than the differences measured by DxO Labs. When both were tested on the 7D, the Tamron clearly did better at f/5.6, but the Sigma compared better at wider focal lengths and apertures than when tested on the 50D. The Tamron also tested slightly better in terms of CA and Vignetting, but also had slightly more pincushion distortion at 17mm than the Sigma at 18mm.
The significant differences between the two lenses
- Image stabilization. The Tamron has it, the Sigma does not.
- Macro capability. The Sigma is laberled Macro, but that should be taken with a grain of salt. MFD is 7.9", MM is 1:3 (.33). The Tamron has an MFD of 11.4", MM is 1:4.8 (.21). So while not really a true Macro with 1:1 or even 1:2 magnification, the Sigma does much better than the Tamron in this regard.