I have had both and to me the 1.4 was worth the extra, it was sharper and had better contrast right up to f5.6, focus is a lot faster and more accurate, the 1.4 also has FTM focusing and a 8 blade aperture giving better Bokeh.
- I found the build quality and noise from the 1.8 awful also the IQ on the 50D left a lot to be desired.lol I forgot the mention the growling sound of the AF motor- I found the build quality and noise from the 1.8 awful also the IQ on the 50D left a lot to be desired.
I have to be completely honest, for me the 1.4 is simply not noticeably sharper than the 1.8 in any situation including test shots. I found them completely equal.
You can list many reasons why the f/1.4 is better, and why shouldn't you be able to...it is 3 times the price. Yes it is better built, focuses with better speed and is less noisy. I would not say it focuses with any more accuracy. All of these things are nice yes, but for 3 times the price you are effectively having "nice to haves" when the resulting image quality will be so similar you won't notice the difference imo.