I recently bought the Sigma 8-16. I think the first question though is why do you want to go 'wide'? & How wide do you want or need to go?
The 'Thing' with UWA's is that the extra wide can tend to pack extra 'boring' in the frame, much more easily than extra 'interest'; they can be tricky to exploit and make work for you.
The Tokina 11-16 was on my short-list. Its very well regarded, its very well rated, and its well priced and considered very good VFM.
In my eventual purchase reasoning, I passed it over simply because it didn't really go so 'wide', and while it may be a 'better' lens as far as optics and construction, for the amount of use and standards I require, that was of little great concern, and spending so much on a lens anyway, so was any 'saving'.
The 11-20 didn't really figure in my buy decision; the extra at the tight end is in the over-lap with kit, so of marginal advantage, while I am suspicious that that extra zoom-range is at expense of compromise else-where, and with that lens priced so close to the 11-16, that is so highly regarded, probably not such a great lens or such good VFM.. but merely my suspicion & gut reaction.
My ultimate choice came down to the Sigma 10-20 f4.5 or the 8-16
By my reasoning, the 10-20 was the best value for money, and the cheaper option, and probably the more usable; the 8-16 was the more expensive and probably less usable, but in-for-a-penny-in for a quid; it went all the way, and goes as wide as you can go.
Yet to get any significant opportunity to play with it, but, so far I'm comfy with my buy..My main concern was getting whole motorbikes in the frame, when parked in tight restrictive spaces at shows and meets and the like, and you have to get up close and personal to get a shot; for that, the added 'wide' of the 8-16 is a little questionable, from few test snaps I've tried on my own bike, (the bike season not yet started really) at the extreme 8mm end, shots were a tad 'wobbly' with distortion; not as bad as with my fish, but of that ilk, and I was getting as much in the frame at 10mm and only a few inches further back, with less skewing, implying . I think I would have been more than happy with the 10-20, and had cash to buy a 35/1.8 prime into the bargain (I just nick the one I bought my daughter atm!) The Tokina, I think I would have appreciated, but regretted it didn't go wider, but I also suspect that limitation would mean I got more better shots, and spent less time zooming in and out, and canning around the frame, looking for distracting details, and just got on with the job!
Which takes us BTT and why you want to go wide and how wide do you think you need go? And as ever, how deep are your pockets!
The 11-16 has reasonable size filter thread 77 I think (82 I think on the 11-20) and it is pin sharp. It's a relatively big lens anyway for DX, I believe the 11-20 will be bigger and heavier.
