I own the Sigma 10/20 and love it! I'll be in Paris in a few months myself, can't wait!
How is the build quality on the sigma? A few things that are turning me away are that I hear that Tokina has fantastic build quality and that the sigma vignettes.
Not sure where you read the vignetting part from, I've never had that issue with mine at all. Build quality is solid, no complaints from me. When I was searching for UWA, everyone said Tokina 11-16, but at the time I had a d3000 and that lense would not AF on my body. Now with a d90, I probably would have given the 11-16 a good look. I can't complain though, I got my Sigma for 425 bucks NEW this past christmas.
Example shot:
http://farm6.static.flickr.com/5128/5362648310_b47b8e78a7_b.jpg
I rarely shoot above 10mm lol.