What is your intended output?I would get the TS-17 (Actually I do have it!)
I know a world leading (in his field) pro photographer and his primary lens is the Sigma 12-24, he absolutely swears by it.
I have the 17 and it is without equal in the ultra wide stakes, even if the Zeiss is fractionally "sharper" the additional functionality of the TS-E make it a far more accomplished lens.
As you said it has no equal. Period!