Not much different really, but Sigma seems to be slightly on warm side.
Distortion is pretty good on Sigma in outdoor shots where the subject is not so close to you.
If for example I am using the Sigma 12-24 on internal shots, I can see the wall is actually curve (distorted) pretty badly as compare to KM version.
Main advantages of the Sigma 12-24 is you can use it on film...
KM 11-18 is much lighter than Sigma version, but don't exactly feel cheap... A lot of plastic... Its ok I guess.
I got both about the same price, with Sigma only $100 more ex.
I am please with both lens, but Sigma lens will be mainly for backup lens. I am selling my 15-30 EX since I am not going to use it. Anyone interested, let me know.
Hart