The Canon 135 F2 L is, IMHO, one of the best lenses they make..
I wonder why you would bother to adapt the Zeiss ?
I agree that if I were a Canon user, I would stick with the 135mm f/2L USM (used it in the past when I was a Canon shooter), even though the Zeiss out-classes the Canon L, for overall quality on Full-frame and for sheer acuity. Having used the 135mm L in the past, I agree that *among the other lenses* in the Canon lens range, it is one of the best lenses for portraits at the 135mm FL.
On the Sony A900, obviously, the Zeiss Sonnar is stabilized - an advantage not present when adapted for use on the Canon 1D body. Neither is the Canon L stabilized on the Canon body - thus this consideration is a wash.
Since it is natively made for the Canon mount, I personally would stick with the Canon L, if I were a Canon shooter, regardless of the higher quality of the Zeiss images and the 24% larger aperture size of the Zeiss (over the Canon L)......primarily due to its total and complete compatibility with the Canon body.