I've owned both and it really depends on your needs. the 50 1.4 is a great lens, but isn't really sharp and contrasty until f/2. The 1.2 is contrasty at 1.2, and sharp by 1.4 or so, but the 1.4 is actually sharper beyond f/4. The 1.2 resists flare much better if you plan to shoot into the sun or shoot a lot of backlit stuff. The AF is better on the 1.2 but not was fast as other USM lenses, but it is more durable than the micro-USM on the 1.4. Distortion and CA are pretty close, but the 1.2 has a lot of LoCA between f/1.2 and 2.8. The build quality of the 1.2 is MUCH better if toughness is a concern. The bokeh on the 1.2 is quite a bit better too, and at 1.2 it's pretty amazing. Is it that much better than the 1.4? Dollar for dollar, no, but if you use it a lot and those differences are meaningful, yes.
Okay, now I wish I hadn't sold my copy of 1.2