I rented the 1.4 for a long weekend and then later I borrowed a spare 1.8 from a friend. I wish I had them both at the same time to do some proper A/B comparisons, but it’s worth noting that in absence of a controlled apples-to-apples pixel-peeping experiment, I’m hard-pressed to see any obvious quality differences whatsoever in the results. Granted, the usage was pretty casual and non-technical in both cases, but it’s still interesting to note. Ultimately, I went with the 1.8 and put the extra cash toward some speedlights.
Build quality of the 1.4 is head and shoulders above the 1.8, no doubt there.
OP, if you’re posting such a question on an internet forum, I’m going to go out on a limb and assume you’re fairly new to the hobby. Unless money is no object at all, just get the 1.8 and learn what you can with it before spending 4x as much on the more expensive gear. Heck, you might find that you don’t even enjoy using 50mm prime lenses.
Most of the time, I see merit in the old adage “buy cheap, buy twice” , but in this case, even if the 1.8 falls apart in 2 years (which it shouldn’t if you take care of it), the lessons you can learn with it and the very decent quality photos it’s capable of make it well worth the paltry sum of $100 brand spankin’ new (with a factory warranty).