DJ, did you consider the Sigma 12-24 as well when deciding on the purchase of the 10-20? It would be interesting to see what the differences are.... (other than price and the focal length)
The 10-20 EX is the newest wide zoom from Sigma, and is the DC designation - which basically means it's designed for the smaller digital sensors (won't work with film). I never really considered the 12-24 as a result. Also to keep in mind the 10-20 takes front filters (which I will utilize), is slightly lighter and smaller (comparatively), and has a shorter minimum focussing distance (maybe not a big priority with a wide zoom, but you never know... I'm finding neat new ways to use this lens).
If you can, make the trade. The 4mm is great to have, and you really can see the diffrence. If you can, go wider. Personally, primes in the wide area tend to work much better than zooms, and why not? They usually do. Wide zooms tend to vignet and distort like crazy, so if you can, get the 20mm.
