I have the Sigma 10-20mm that i use on both the a700 and a350, its a very good lens.
Although i have been thinking lately to change to the Sony 11-18mm due to having DxO, i would like to take advantage of the lens correction modules they have for that lens.
Only thing i would say about the Sigma, is don't go for the fixed f3.5 version, its a lot more expensive and i don't think you would ever gain any benefit for having that aperture anyway due to most times wanting the lens to be stopped down to get the sharpness.