It is actually a tough call. I have Sigma 10-20 and like it, great to use with my expensive (>100$) super-dense ND filter B+W ND110, but the newer 8-16mm is better optically (3 and 4 stars from photozone.de for 10-20 and 8-16, respectively), and is much wider. IMAGE LINK: http://www.flickr.com/photos/syamastro/4971058777/ A way to oblivion
Recently, I haven't been using the ND filter much with 10-20 - I think because as my landscape shooting skills improve, I make an extra effort to do the shooting at the right time (usually around sunset and sunrise) when ND110 filter is not needed. I now consider the ND110 as a "lazy photographer substitute for proper landscape shooting". But even at sunset/sunrise, there is usually too much light to get long exposures (>30s) without a filter; ideally one would need something like ND8. So 10-20 still could be the better choice for landscape photographers.
Simply compare the shots of comparable objects (piers in lake Ontario), first done with Sigma 10-20 in the "lazy" way (ND110 filter; day time), and "proper" way (sunset, light filter, ND4):
, on FlickrIMAGE LINK: http://www.flickr.com/photos/syamastro/7756241442/ Golden hour reflections
, on Flickr
(BTW, the second shot was not even made with UWA - I used my Sigma 17-50 at 17mm).
Now, as I looked closer at photozone.de resolution numbers for both lenses, I am not even sure if 8-16 is indeed better. It is definitely better wide open - but how often will you use it wide open? For landscape purposes, I find myself using f/11 most of the time, and according to photozone, both lenses have comparable resolution at this aperture (both are in the "very good" territory; BTW, the Canon's 10-22 is worse at f/11 and 10mm than both Sigmas - it is mostly in "good" territory).
And my feeling is 8mm will be simply too wide for me - the near vs. far will be so much exaggerated, that it would be pretty hard to find a good setting for a nice 8mm landscape shot. When dealing with such extreme angles of view, I'd rather use my Samyang 8mm fisheye (gives even wider angle than Sigma's 8mm, because of the fisheye projection; and actually distorts shapes much less than rectilinear 8mm lens, which dramatically stretches objects near the frame edges).