I've actually changed my mind after reading this review an looking at the sample pics:
http://www.juzaphoto.com/article.php?l=en&article=34
I had decided on the Nikon 10-24 based on the slightly longer zoom range and looking at "reviews" .
After looking at the above I've decided on the Sigma 10-20 f3.5, this came up better than the Tokina which is pretty much universallly regarded as a good lens, and I figured the difference in price would allow me to get closer to buying a Nikon 35mm f1.8 prime for low light as well.....
cheers
Gary