The 50/1.4's 8-bladed aperture gives marginally smoother background blur than the 50/1.8's 5-bladed aperture... If you look at the reviews, some call the difference in bokeh huge... others call it moderate or minor. Decide for yourself. Yo'll find some useful comparisons here and here.
One thing is clear... the f1.8 lens' background blur is coarser (busier, more nervous) than the f1.4's.
In many respects, the image quality of the 50/1.4 is marginally better than the 50/1.8... In addition to nicer bokeh, it also gives richer, better saturated colors. This seems to be at least in part because it's more resistant to veiling flare that costs both contrast and washes out colors. This suggests better coatings and/or better light dampening in the 50/1.4. On average, the 50/1.4 gives ever-so slightly higher resolution, too.
The differences in image quality are there and are in favor of the f1.4, but are fairly subtle. The main differences are in the two lenses' build quality and "feel"....
First, just physically the f1.8 lens is as plasticky as a lens can get... even the bayonet mount. We've seen photos of more than one of them here on POTN broken in half... sometimes just during normal use. The focus ring is very narrow. There's no focus distance scale. To fit the Canon lens hood, you need to first buy an adapter... but the front element of the f1.8 is fairly recessed, so a hood might be less important than some other lenses.
The AF of the 50/1.8 is about as basic as it can get. Folks who try to micro focus it usually give up because it's so erratic, it's next to impossible to dial in a "correct" MA adjustment. It's also slow, noisy, and tends to hunt more.
The AF on the 50/1.4 is a hybrid form of USM... Also not perfect. It's a bit fragile. But it's far more accurate, quieter and faster than the f1.8's. The f1.4's AF might break down due to this fragility... Is that better or worse than a lens that breaks in half?
This is not to say that the 50/1.8 II is a "bad" lens. For what it costs, it's pretty amazing. Someone who needs a 50mm for occasional use or is exploring using faster primes for the first time might find it more than adequate for their needs.
For someone who wants a lens that will hold up to more regular use, simply feels better built, focuses faster and more precisely, and gives marginally better IQ, the more expensive 50/1.4 might be preferrable. It's still one of the cheapest primes in the Canon line-up.
And if neither the EF 50/1.4 or EF 50/1.8 II impress... Well there are plenty of alternatives: EF 50/1.2L, Sigma 50/1.4, Zeiss 50/1.4 ZE (manual focus only), and literally millions of vintage manual focus prime lenses that are easily adapted for use on Canon (manual focus and manual aperture only). There was also the old EF 50/1.0L... (good luck finding one!) and the first version of the EF 50/1.8 (which usually sells used for more than the Mark II, has metal bayonet & focus scale, but not a whole lot of difference in other respects).