It's comparing apples and oranges isn't it? But I do like these kind of questions. I just tend to think: what would I rather own or use? I had a T90 for a while with a 85/1.2L in FD mount, but I didn't like the soft portrait rendering at f/1.2. Compared to my contax G lenses it just didn't seem sharp at all! It's unfair off course to compare such a fast lens at it's maximum aperture to a slower lens at it's maximum aperture, but I kept the gear I liked best. I make better portraits with a 135/2.8 for some reason.
If you are mostly interested in 'available darkness' photography, there are some points on which one can compare the two lenses. I would go for the 35/1.2 on the leica because it allows for longer shutter times because of the lack of mirror slap and the shorter focal length. Also it has more depth of field than the 85/1.2. The 85/1.2 at short distances doesn't even allow for the focus-recompose technique. The depth of field is too small; once you recompose, focus is off.
I think you could shoot the 85/1.2 at 1/60 of a second at f/1.2. That would be your maximum low light ability with that lens, if you have a steady hand. I would rather shoot a fixed lens rangefinder with a leaf shutter at 1/15 of a second at f/2.5. it would give me the same low light ability, more depth of field, and better performance optically. In fact, if you shoot leica, why not use a 35/2 summicron? I'm sure it's better corrected than the 35/1.2. Do you really need a 35/1.2 and super fast film like delta 3200? To me, superfast lenses are a compromise on performance. I like the crispness of their slower siblings