Why spend more?
With the 50mm f1.8 lens available for less than a hundred dollars why spend so much more to get the f1.4? The answer is you may not need to. It all depends on your seriousness budget and how long you need your lens to last.
If you want a "starter lens" for shooting at 50mm (or with prime lenses in general) the f1.8 would be a great buy. 50mm is a very useful and intuitive focal length to spend some time with because it sees the world at the same distance as your naked eye (regardless of your camera's crop factor). So you could buy the f1.8 cheaply regard it as a "play with it" lens and get a nice introduction to "prime lens quality." The f1.8 will seem like a substantial step up from kit lenses and most consumer-priced zooms and amazing bang for few bucks.
So if the f1.8 is such a great bargain why would the f1.4 be among Canon's most all-time popular lenses? It's that the f1.8 can take the great shot within certain conditions but the f1.4 delivers within a much wider range of conditions. In other words "You get what you pay for" and we'll save the best for last.
Affordable-but-Solid Contruction: The f1.4 will likely have a much longer life than the cheaper plastic build of the f1.8 and retain more resale value. It's an investment rather than a commodity. And it'll be more certain on your camera and in your hand. (My first one finally needed some calibration after 80000 shots and extreme wear-and-tear from frequent swapping with my other primes.) Users sometimes report the front glass falling out of their f1.8s. For the f1.4 the main issues revolve around the Micro USM focus motor which is not as sturdy as true USM.
Focus Versatility: The f1.4 lets your camera autofocus and then lets you tweak further by hand without flipping a switch - that's called "Full-Time Manual Focus." The f1.8 requires switching back and forth between auto and manual focus. The f1.8 is famously noisy/buzzy during autofocus has a bare-minimum focus ring and no distance scale. The f1.4 will autofocus more reliably especially in dim light though it will fail occasionally when starved.
Resistance to Abberation: Chromatic abberation (fringe colors) and barrel distortion are evident-but-low for both lenses at wide apertures - that's "prime lens quality." But in comparison tests the f1.8 is more susceptible to vignetting (shadows around the corners) halation (glowing around the highlights) and lens flare. For instance lens flare within the f1.4 tends to be more tightly controlled - "in focus" - whereas a bright light source is more like to blow out the whole shot in the f1.8. All these factors improve when stopped down but lag about a stop behind the f1.4.
Color: However if the f1.8 catches up at f/8 to the f1.4 by many standards it rarely catches up to the f1.4's saturation. The f1.4 has "proper-to-strong" color richness at all but the widest apertures while the f1.8's shots are much more likely to require postwork. (I do however get better saturation from my 24mm f2.8 and 100mm Macro f2.8. The 50 f1.4's saturation seems good-not-great by comparison.)
"Headroom": The engineering of both lenses lets you choose the tradeoff between "most possible light" or "most possible clarity." It's by design that you can choose "more light for less crisp" or stop down for sharpness. *Samples vary* but the average 50mm f1.4 should consistently "get down to sharp" more quickly "sharp enough" by f/2.0 "very very sharp" by f/2.8 (often exceeding the professional 24-70mm f2.8 L when wide open) and delivering "unreal sharp" by f/4. (I saw insane "specks of mascara sharpness" at f/3.5 from my first f1.4.) Again the f1.8 will probably lag about a stop behind that curve.
My second 50mm f1.4 performed even better than my first right out of the box "marginally sharp" at f/1.4 and increasingly beyond reproach by f/1.8-2. (At f/1.4-1.6 it suffers only from halation and some light fall-off in darker areas.) So if extreme sharpness is necessary for you shop with a strategy that will let you return your lens or get it calibrated if not up to your needs. My guess is that my first one was more typical out of the box but it approached the performance of the second after calibration.
(It's also worth noting that the premium-priced 50mm f1.2L is drastically more sharp (and better performing generally) at wide apertures but *less* sharp at f/2.8 through f/8. The f1.4 is a better "walkaround" performer than the f1.2L lens that costs four times as much.)
Regarding light return specifically my own experience in lens-swapping baffled me until I read other reports that the f1.4 exposes a third of a stop brighter than most other Canon lenses. It's brighter in the viewfinder generally and really IS a whole stop "faster" than the f1.8 at maximum apertures (i.e. the same net exposure at half the shutter speed). If you're willing to sacrifice some clarity that extra stop can make a huge difference when you're challenged by moving targets in low light.
(For instance shooting "wide open" for performers in dim venues. Faster shutter for less motion blur. More light for better color. And the edges may be soft at 100% magnification but *relatively* clear compared to the out-of-focus background. That "illusion of clarity" isn't as likely to print very well but resizes very snappily for the web.)
So the f1.8 can certainly produce some stunning images particularly in general daylight photography OR tightly-controlled conditions OR stopped down but is less adaptable to challenging circumstances that the f1.4.
"The Best for Last...":
Now with both these lenses you get the advantage of marvelously wide aperture which can be used for a tight focal plane that lets the background (or foreground distractions) fall quickly out of focus. This is of course a cornerstone of creative photography and both lenses give you plenty to explore. (In practice even f/2.8 delivers a pretty shallow depth of field in close-up shots so these wider lenses give you even more room to play.)
However there is such a thing as "blur quality" called "bokeh" based on the number of aperture blades within the lens. The f1.8 has five and the f1.4 has eight. The f1.8 will portray out-of-focus lights more pentagonally the f1.4 more roundly. (In focus those same lights will be eight-pointed stars with the f1.4 ten-pointed with the f1.8 - odd numbers of blades double the number of points.) But most importantly the blur from the f1.8 can be rather "choppy" especially at wide apertures while the f1.4's is consistently more "buttery smooth."
In other words there's more to quality than sharpness - there's also quality where your shot is LESS than sharp. And this is where the f1.4 becomes "a favorite lens" for some people even at over three times the price of its diminuitive counterpart.
Make no mistake the f1.8 would make an excellent "starter" lens. But the f1.4 is an exceptionally *serious* lens. Are you still learning to love photography? Then $80 is a fine price to pay for a lens you might outgrow. Or do you already love photography? Then $300 is a worthy price for a true investment that will reliably pay off. So they're both bargains just buy what's best for you.
(Addendum - Canon also sells a 50mm f2.5 Macro lens around $250. If you NEED macro it's reportedly pretty good and for general purpose as well. But it's a) not even as fast as the f1.8 b) more difficult to manually focus than the f1.4 and c) not as creamy in the bokeh with six aperture blades instead of eight. And Canon's 100mm version is drastically more practical for macro work and better performing generally. But the 50mm Macro does become a contender at a "middle price" if what you really need is one decent lens to do as many different things as possible though none of them as well.)More detail ...
