Hi Bart, that's a great question, but one for which I don't currently have a definitive answer as I've not tested the f1.4 version. The f1.8 is certainly very good for the money and you can see resolution results with it in our 10 megapixel group test here:
www.cameralabs.com/features/10Megapixel ... ge4b.shtml
As for the build and other aspects, you've really answered that for yourself. The f1.8 is a budget starter lens, whereas the f1.4 is a higher-end 'upgrade' model with superior build quality, faster and quieter USM focusing, a proper manual focusing ring and of course that faster aperture. If you're really serious and can afford it, I'd go for the f1.4 myself. But the f1.8 is great value for anyone who wants to complement a standard zoom with something better suited to low-light photography.
Or you could remortgage and get the 50mm f1.2 L!
Here are some links to USA prices anyway for comparison...
Canon 50mm f1.8
Canon 50mm f1.4 USM
Canon 50mm f1.2 USM L
Does anyone have any of these lenses and care to comment?
I'm also considering doing some reviews in the future of fixed focal length lenses. Which Canon models would everyone be interested in seeing?