The 1.4 supposedly has better build quality but breaks more often and is not as sharp.
At the middle aperture values, maybe. At f/2 or wider, though, the f/1.4 is far superior in both sharpness and bokeh quality.
Thats about right, not a vast difference. They are close and after f4 they go back and forth depending and if you are looking at center or edge. Either one is likely to be the sharpest lens in your bag. you would be hard pressed to pick out the difference except for the bokeh, which is better on the 1.4. Both are sharper than the 1.2L. I was going to upgrade to the 1.4 but the old 1.8 has the metal mount like the 1.4. The 1.8 feels flimsy but can take a hit. I know of 2 photographers who bumped into stuff with their 1.4 and it stripped the nylon gears or something in the IS so I'd sit tight with the 1.8. With todays sensor you can shoot in pretty low light so the difference between the 1.2, 1.4 and 1.8 is slight. I'd sit tight. I suspect, without a shred of evidence, that they will offer something new in 50mm range. Probably an IS f2. If it was super, super sharp I might be tempted but with Canons current pricing I can thing of better things to do with the $700 bucks it will cost. The really good news is, not matter what 50mm you have in your bag, it;s the sharpest lens you own