Sigma has 2 10-20s - one is f3.5 HSM, the one being f4.5-5.6 HSM
The siggy f4.5-5.6 has a significant curvature of field, and strong barrel distortion at 10mm with a rather unusual and complex pattern, localized mainly towards the corners of the frame. Sharpness is good at center, but generally poor at corners.
The siggy f3.5 has rather strong barrel distortion at 10mm too, but far more uniform and easier to correct. Sharpness is generally similar to f4.5-5.6's.
The tokina 12-24 has strong barrel distortion at 12mm. Sharpness is generally good with this lens, but CA levels are very high.
If you need FTM, or very wide angles, and don't mind stopping down to get better corner to corner sharpness, the sigma 10-20 f3.5 is a good choice.
If you want good sharpness wide open but don't mind high CA levels, the tokina is a better choice.
If you don't mind considering other choices, there's sigma 8-16 - it performs far better than both sigma 10-20, but doesn't take filters.
The tokina 11-16 is good too - there's a constant f2.8 and bags of edge to edge sharpness, but comes at a price of limited focal length and high levels of CA
Hope this helps.