I am an owner of 85L II and also have shot with the Sigma 85 F1.4 on both my 1DS2 and D3 many times, here is my take on those:
Both are great, the F1.2 max. aperture is obviously the unique feature of the Canon, if you need/want that aperture, there is no other alternatives, it offer great bokeh, sharp wide open performance, great contrast, top notch build quality, however, it focus very slow, it's by far the slowest AF focusing speed lens I have, it's big and heavy, well, some people call that a "con" but for me that's a "pro", I love large and heavy lens, it balance very well on the 1DS series body and gives me a very steady shots.
The Sigma is really almost as good except it doesn't do F1.2, great center sharpness wide open, focus much faster, some people complain focus consistency with the lens, but the 3 I tried (on my D3 and 1DS2) didn't have any focus issue, and also per the owners of those lens, they all work fine on their D700, D3X, 1DS3, 1D4, it's a much smaller lens, so it may balance better on the smaller camera bodies, I put that on my 5D2 once and it feels about right for that size body.
It really depends on your own shooting style and requirement, it's very hard for us to tell you it's worthy or not to spend twice as much for a L instead of the Sigma, for me it's worth it since I love the F1.2 and it's solid build quality, I use F1.2 quite often, and my 1DS3 friend however though the Sigma is good enough for him as he prefer the F1.4-1.8 DOF anyway, so he doesn't really care for F1.2..... so it's all personal preference.
Another question you should ask yourself is how are you going to use this new lens, mostly wide open or stop down? the reason I am saying that is I ONLY use them at large aperture, not necessary always wide open but always at relatively large aperture, say from F1.2 to F2.0. And for stop down landscape application for that focal length, I don't even touch those 85L or Sigma 85, both lens have a pretty bad edge/corner performance no matter how you stop it down, even the cheap 85 F1.8 beats it in that area, of course that's depends on what you are comparing it to, and in my own case, I am a TSE and Zeiss prime lens landscape shooter, so for that focal range I use the Zeiss 100 and sometimes even the 70-200 F2.8 IS MK2 for landscape work instead, so don't buy those large aperture lenses for landscape just because they are expensive and have red ring.