The FD version is a little gem. much smaller than the EF, and I think sharper at the focus point. By many accounts, it was considered the better performer. There is one comparison out there of this on the a7:
Sony FE 55mm f/1.8 Z vs Canon FD 50mm f/1.2 L | Viktor's photo blog
Not a terribly good comparison, and it fails to test the 1.2 for the real reasons people would be using it, but nonetheless...
Shooting LF really makes a person much more critical of these small format lenses, what with all their retrofocus designs and the accompanying optical compromises they force on the shooter. It gets much worse with zooms, too.
My goal is to achieve a more LF look to my digital images, and the slight bit of swirl in the Mitakon may help achieve that.