Even the MTF charts you posted show the f1.2 to be a lot better than the f1.4 when "wide open" (maximum aperture). That's one big factor when looking at a wide aperture lens: is the quality good enough to actually use it wide open?

Also, I have the f1.4 one here, and build quality is crap. It has a "USM" motor, but not the highly desired "ring USM" style of motor. This thing is noisy, slow, and generally a bad experience.

It's been back to Canon for repairs twice, and still gets stuck sometimes when trying to focus. AVOID.

EDIT: Apparently the real bargain for a 50mm prime lens, is the f1.8 version. Much, MUCH cheaper, but about the same real-life image quality as the f1.4 lens.

Cheers