It's something I'll be considering when I find out whether or not my current 50mm is able to be fixed cheaply enough to justify it... which is why I've been reading about this very same sort of issue today.
Now, given that you're Canon and I'm looking at Nikon it may or may not apply. But from what I've learned today the Nikon 1.4 version becomes sharper by about f/2.5 than the 1.8 version. So what that means to me is sharper at wider apertures rather than the extra fraction of a stop.
Of course, I've also heard that especially with the Canon the build quality is much better on the 1.4 but I'll leave that to people with Canon experience.
So there's my 2 cents worth
Oh, and I'll pop this over to the Canon forum so that it's more obvious that you're looking for a Canon lens.
