I owned both lenses and used them on my 5D Mark III.
The 50L comes with weather sealing, generally feels much much more worthy than that over 20 year old 1.4 and has a beautiful bokeh, but it's not the sharpest tool in the shed. Below f/2 the optical performance is mediocre, it's quite soft in the center and even worse in the corners. The 1.4 performs almost equally but the bokeh is not that beautiful, don't get me wrong it's still great.
They both perform very good from f/2 or f/2.8 upwards, the 50 1.4 is very sharp at f/4 but if you want to shoot at f/4 you wouldn't need a f/1.4 or f/1.2 lens in the first place.
If you don't care that much about tack sharp pictures like a 100L or 135L deliver, they are both good performers, it's your choice if weather sealing and that extra half stop is worth the quadruple.
I know that pixel peeping makes no one happy but the comparison shows the difference clearly:50L vs. 50 1.4 @ f/1.450L vs. 50 1.4 @ f/4.0
I now finally gave up my hope for a good 50 with wide aperture, bought the excellent Sigma 35 1.4 Art and hope for a new 50 1.4 by either Canon or Sigma (Art series) that turns up the optical performance by many notches.