Caveat: I have never used a Sigma 50 f/1.4, so I can't say anything about it. I do use the Canon 50mm F/1.4, though, and it suits my needs just fine. I've had to learn a few things about it through trial and error, and so will anybody who uses it for any length of time. Although many beginners start out with it (it's a cheap-ish prime with excellent IQ), many of them also get frustrated with it because of it's finnicky nature. Learn how it autofocuses. Know that sharpness is ok-but-not-great at f/1.4, but gets really, really good when you get into the f/2-f/8 range. Microadjust if you have the ability to, and learn how it responds to you and your camera body. Once you do that, I think you won't want another 50 (unless it's the 50L, of course). It's by far the lens that I own that has the biggest bang-for-the-buck value, and I own two L lenses.
