Peter K Burian has done some tests comparing the the Sigma AF 10-20mm, Tamron SP AF11-18mm, and Tokina AF 12-24mm.
“Sigma AF 10-20mm f/4-5.6 EX DC HSM: The images also exhibit great clarity and impressive definition of intricate detail. This Sigma lens seems to be optimized for the very finest results at focal lengths from about 10-14mm at most apertures. By 15mm, edge sharpness is a bit softer, although central sharpness remains very high.
Tamron SP AF11-18mm f/4.5-5.6 Di II LD Aspherical (IF): My best images—made at any focal length at f/4.5 to f/11—exhibit remarkable contrast and clarity, plus moderately high sharpness and resolution of intricate detail. This consistency indicates a particularly effective optical design.
Tokina’s AF 12-24mm f/4 AT-X PRO DX: The Tokina 12-24mm zoom produced remarkably high sharpness and resolution at every zoom setting—particularly at longer focal lengths—at all apertures from f/4 to f/11. Consequently, I never hesitated to shoot at f/4 for fast shutter speeds in low light. Performance at f/16 is not as stellar but still acceptable. After selecting my technically best images, I was able to make absolutely superb 12×18” prints that look sharp and finely detailed even under close examination. “
See also: Lens Reviews | Sigma Lenses | Tamron Lenses