(but it won't work with a D300...).
This is incorrect. The Nikon (or Tokina) 12-24mm will work with a D300, just not the D3. The D300 is still a cropped sensor like the D200/D100/D70/D80/D40/D50.
At least for the Nikon, the 12-24 will work fine even on full frame. At some point in the zoom range (guessing around <15mm) it will start to vigenette but otherwise it works great. This is one of the few DX lenses that has a substantal usable range on a full frame camera. Unlike Canon lenses, Nikon DX lenses function and fit perfectly on film or full-frame cameras. If you have a DX camera there is no future proof solution that gets you as wide that can be used UW. (The Sigma 12-24mm is too thick for most domes).
If it were my money I'd get the Tokina. The quality difference is small and the cost is large. For $500 you could get the 10.5mm or 10-17mm too!
I bought the Nikon when it was the ONLY WA zoom around. Now we have choices. Its a nice lens.