The difference between F1.4 and F1.2 is some where between a 1/3 to a 1/2 stop. It's a significant and worthwhile difference but an F1.2 doesn't become much more difficult to focus or use than an F1.4 lens (not withstanding some possible further softeneing of the image when wide open which makes manual focusing a little harder still). I'm a big fan of fast lenses and will always choose a fast lens over a slow one, for many reasons, and you can always stop down a fast lens if you need to.
JJ