Well I like the wide end of the focal length range, and can recommend both the 10-17 as well as the 12-24. Both are excellent pieces of optical glass. The two lenses actually complement each other very well. To understand this you have look at each of the lenses' field of view.
10-17 180 degrees to 100 degrees
12-24 100 degrees to 60 degrees
Obviously the difference is the 10-17 being a fish eye, in that the focal lengths overlap, but that is miss leading.
The other item is the quality of the 12-24. An evaluation on dpreview
http://www.dpreview.com/lensreviews/..._p15/page3.asp rated this zoom superior to the new DA 15/f4, which is unheard of.
"Unfortunately it (the DA 15) doesn't come out so well against the excellent Tokina AT-X PRO SD 12-24mm F4 (IF) DX, which we'd expect to perform near-identically to Pentax's own smc DA 12-24mm F4 ED AL (IF) - in this case the zoom is sharper, and has lower falloff and equally low distortion, although it does show more chromatic aberration."
In terms of CA on the 12-24, I have never had a problem with it, and I have shot directly into the sun and many of sunsets (also tried it during the day). It can be there, however its well controlled and can be removed with software. Essentially the Tokina 12-24 is the same lens as the Pentax (the probable difference is the Pentax SMC coating and the external design/grip, along with the AF clutch).
In terms of the FE image quality, it is excellent, also. Neither Canon nor Nikon has a zoom FE, and in terms of wide angle - just primes. The 10-27 is another collaboration with Tokina. It is an excellent lens, fish eye at the 10mm end, however when at the other end - 17mm you have to stretch to see it (however it is still there, just subtle). The image quality is superb. The focal length of the range is so short, that it actually acts like a prime. Also the build quality is beyond compare.
Between the two lenses, you have a continuous Field of View range from 60 degree, all the way to a full 180 degrees (and watch where you put your toes, they may be in the frame when you take the shot).