Of the two the f/1.4 is the better lens. The EF 50mm f/1.8 II is built like a toy and has a tendency of breaking into two pieces with the slightest bit of mishandling. The EF 50mm f/1.4 has a much better build, USM (micro) focusing motor, 8 blade aperture (5 on the f/1.8), and a better manual focusing ring.
If you end up with the f/1.4 I highly recommend purchasing a lens hood, it can help protect the fragile micro-USM.
I use the 50mm f/1.4 for indoor low light shots, usually half body portraits.
It has excellent image quality and a close focus distance of about .45 meters.
I am very pleased with the lens, have considered upgrading to the 1.2 but can't think of a reason why.
Never had a problem with the IQ of ability of my 50 1.8. It's my studio portrait lens so I don't have to worry about bokeh shape or focusing in low light. It is one heck of a bargain IMO. It is built like a toy and I doubt it would survive a drop, but I haven't dropped it or any lens so it has more than earned its keep. For $100 it's been a steal.
If you need a better build or something sturdier, the 1.4 is definately the way to go. If not, the 1.8 is a great bargain.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.