What for? I like the bokeh of the 1.2 better, but it's a matter of taste and you have to know what you like better. Sharpness-wise I don't think they're all that different, at least as far as I'm concerned the 40/1.4 has left little to desire. Forums, flickr etc. are full of pictures from both lenses.
In my opinion the 1.2 is better, a lot bigger and heavier and certainly something like three times more expensive. If, after looking at pictures, you think you get three times the enjoyment out of your pictures, go for the 1.2. My 1.4 is an everyday walkabout lens, the 1.2 certainly wouldn't feel like that.
It also depends on your camera; on my M5 the 1.2 feels just fine, on a smaller camera it might feel too bulky, and if you have a Bessa or something, or a .58 Leica, you have slightly more trouble focusing the 1.2 accurately depending on your eyesight. Nobody, and no online subjective judgments in particular, can replace taking the lenses in your hand and trying them out.
I like both better than the 35/1.4. 😉