I went through similar debate before I ultimately settled on the Sigma 10-20 f4-5.6, and I included the Tokina 12-24. I will say, that an UWA opened my eyes to critical composition around the characteristics of the lens, and is probably one of the most difficult lens to use. One thing that helped was turning the grid on in the D90 viefinder, to square up verticals and horizonals when composing the picture, and minimize post corrections.
Tokina 11-16. Reportedly a great lens, but I ruled it out due to its short range. In retrospect, I am glad I did as I am taking more shots than I anticipated towards the longer end. F2.8 was tempting, but F4 (at ISO 1200) has not been limiting as you will note in some examples later, most taken with available light.
Tokina 12-24 vs. Tamron 10-24. These were stocked locally and I took a series of shots inside the camera store at various apertures and focal lengths, with and without flash. On examination at home and "pixel counting" learned several things. Use a tripod - which I was not anticipating on a non-tele, but it showed. The Tokina IQ was better than the Tamron. FOV is what ultimately held me back from the Tokina, as framing, used a set left point and could judge the difference of 10 vs 12.
Didn't test the Nikon 10-24 because I didn't want to be tempted. No Sigma dealer within 200 miles per their web site so had to order - with a 30 day return. Tested out well and kept it. Both it and the Tokina are very well built. Accidently dropped the Sigma on the sidewalk with a bone chilling hollow thump. No damage - but I wouldn't recommend trying it though.
Most photos on my website were taken with the Sigma. The only exceptions are the telephotos across a lake and a closeup of stained glass in the top of a dome 40' up. Also, my self portrait as the Sigma is in the photo with me.
http://www.imagesforrealestate.com/Interior-Images.html While I particurally like several of them, most proud of is the table with all the mirrors - and I am not in it!
How about a Nikkor 10-24mm f/3.5-4.5 
