I was on the crux of buying the Tokina 11 - 16 mm F2.8 for my 50D, but ended up with the Sigma instead, as I didn't really need the F2.8 for what I shoot. I have used the 16 - 28 and nothing but great results with it (Golden Hour Landscapes though)

But you could try here.

And this.

They always seem to rate very highly, as on dpreview.com as well