I bought the 8-16 last week from park (before sale :bonk: )
Although i havent used the 10-20, I used a 10-24 once before, and didnt really believe peoples reviews until i saw the extra 2mm for myself, opens up a whole world of difference!
As with most UWA it is a little soft at the edges, but much better than most, and better than I expected.
One thing you may consider a downside is filter compatibility.
Lab test results and review...
http://www.photozone.de/canon-eos/515-sigma816f4556apsc?start=1