Outside of the obvious, FF vs Dx and the price, what are the differences in IQ and abberitions? I recently returned a Tokina 12-24, haven't been able to determine for sure if I got a less than stellar copy or if they are all like the one I had, so I'm not willing to rule it out just yet. The FF of the Sigma is interesting, as I still have my F5 and the thought of 12mm on FF has peeked my interest. I do remember reading a review comparing the Sigma 10-20, Tokina 12-24, Tamron 11-18 and Nikkor 12-24 which seemed to favor the Tokina as a best bang for the buck over the Nikon, but the Sigma 12-24 wasn't part of the comparison. (if anyone remembers where that review was, I can't seem to find it!) If cost were no object, the Nikon would likely be my choice, but it's really out of reach considering the use it will get and other lusted after lenses and accessories.