The 50mm f1.2 is sharper and offers some improvement in resolution over the 55mm f1.2.
However, the 55mm f1.2 seems to have the better bookeh.
There are good and bad samples/copies of both lens. But one way to tell if you have a good copy is if the lens is sharp wide open at f1.2.
At wide open, the 50mm f1.2 beats the 55mm f1.2 hands down. The 50mm f1.2 has a very small barely noticable lead at f2.0 as well, but by f2.8, the 55mm f1.2 is slightly better.
If you plan to use either one on your 5D or 1Ds, they sharpen up very well and the images do pop with either lens.
I have both and recall my 50mm f1.2 was not great at wide open. I sent it to Olympus in NY for a CLS and when it returned, it soon became my favorite f1.2 lens.
I also have the Nikon 50mm f1.2, Sigma 50mm f1.4, and the Minolta Rokkor 58mm f1.2. The Rokkor tops the list at wide open, followed by a tie between the OM 50mm f1.2 and the Nikon 50mm f1.2 at wide open. The Sigma gets the points for AF and creamy bookeh.