For what it costs, the 50/1.8 is a real bargain.
The differences between it and the 50/1.4 as far as image qualities are concerned are subtle, but if you test them carefully side by side you can tell them apart. The f1.4 has a rounder aperture (8 blades vs 5) that makes for nicer bokeh. The 50/1.8's bokeh can be rather "nervous" or "busy" in some situations. The f1.4 lens also controls veiling flare better, making for better contrast and color saturation. But both of them need to be stopped down a bit, typically.
The AF system performance of the f1.8 is likely it's weakest aspect. It's slow, noisy, hunts more and is not particularly accurate. Folks who have tried to use popular focus micro adjust softwares with the f1.8 have given up... it is too inconsistent to "dial in" focus with the software.
The AF performance of the f1.4 is much better. But it's AF is sort of vulnerable to damage. It's usually a good idea to get the hood and use it... both when shooting and reversed when storing the lens, to help protect it from bumps that can damage the AF system. Some also feel using manual override of AF a lot tends to accelerate wear in the mechanism. (The 50/1.8 has to be turned off at the switch before manually focusing it via the very narrow focus ring. Failing to turn it off and overriding it will damage the AF mechanism.)
Other durability issues... The 50/1.8 is pretty easily damaged if dropped. It also is lightly built and tends to wear out with very regular use, such as a pro might do. It might be fine for occasionaly use, though. The plastic bayonet mount isn't very confidence inspiring, but seems to hold up okay.
A minor design issue... the 50/1.8 is light and compact, but to fit it with a lens hood you have to buy an adapter in addition to the hood. On the other hand, it's front element is somewhat recessed and shaded, so a hood might be less necessary than with other lenses.
The 50/1.4 has a focus and depth of field scale. The f1.8 doesn't.
I gotta say, my own experience with a 50/1.4 bought used ten years has been completely problem free. I have no idea how old it really is... But it's always been used with a hood and I really don't manually override AF very much. I looked at the 50/1.8 before buying the f1.4, but didn't think it would hold up to my uses.
The 50L is a very nice lens... Solidly built and quality all the way, but of course it's pricey, large and heavy. There have been stories of some issues with focus shift. It seems optimized for portrait or slightly greater distances... and less so for very close or more distance subjects.
The is also the Sigma 50/1.4 to consider. It feels better built than the Canon equivalent... But only time will tell us if it's actually more durable. The Canon has been around for a couple decades now and likely many tens of thousands more of them have been sold. The Siggy is a relatively "Johnny come lately", has been around for roughly five or six years now, if memory serves. The Siggy is sharper wide open and has slightly smoother background blur (9 blades vs 8). It has HSM, which should be fairly equal to Canon USM. For a 50/1.4, the Siggy is huge and heavy. Who ever heard of a 50mm needing a 77mm filter?! (The 50/1.2L uses a 72mm filter).
). it was my first lens when I started out with a DSLR.
You may end up sticking with your nifty fifty.
