Other than looking very differently on the outside, image wise they are all very similar. Personally I'd take the FD 1.2 SSC for the latest coatings and look.
One interesting thing about the Aspherical version, one of my 75k serial number lens copies is 10x more radioactive than my thorium element much sought after Concave front FD 35mm f/2. It makes sense since the chunk of thorium in the 1.2 lens is massive in comparison. My 126k serial number aspherical 55 is not radioactive so Canon must have dropped using it during the design run. That means I really have 5 55 1.2 designs to compare.
I'm not quite sure of your count of 5 55mm f/1.2 since the Canon Camera Museum shows 6, which may also help explain your radioactivity observation.
The ten 5xmm f/1.2 lenses across the FL, FD, and nFD lens lines in chronological order are:
|Name||Intro Date||Intro yen||elements/groups|
|FL 58mm f/1.2 (I)||3/1964||33,800||7/5|
|FL 58mm f/1.2 (II)||3/1966||33,800||7/5|
|FL 55mm f/1.2||7/1968||36,300||7/5|
|FD 55mm f/1.2||3/1971||39,000||7/5|
|FD 55mm f/1.2 AL||3/1971||145,000||8/6|
|FD 55mm f/1.2 SSC||3/1973||50,000||7/5|
|FD 55mm f/1.2 SSC AL||3/1973||147,000||8/6|
|FD 55mm f/1.2 SSC Aspherical||3/1975||80,000||8/6|
|nFD 50mm f/1.2L||10/1980||90,000||8/6|
|nFD 50mm f/1.2||12/1980||52,000||7/6|
|50mm f/1.4 notes:|| || || |
|FL 50mm f/1.4 (II)||5/1968||21,800||7/6|
|EF 50mm f/1.4 USM||6/1993||49,000||7/6|
Note 1: There are 2 versions of the aspheric SSC FD lenses with very different prices but the same elements/groups construction -- perhaps a very different glass (and radioactivity) selection?
Note 2: The nFD non-L lens uses a 7/6 construction, like all the 50 f/1.4 lenses since 5/1968 and unlike all the other 5xmm f/1.2 lenses.
Note 3: I added the early and current f/1.4 to supplement Note 2 and show the yen for price comparisons.
I'd love to get an EdMika adapter or two, though the first lens I might try it on would be my FD 300mm f/5.6 SSC (the one with internal/rear focusing and a 55mm filter).
Keep up the great work ontarian!