Sigma 24-70 f/2,8: Good! (forgive me the PS)

Got the 24-70 f/2.8 (+letters) and found out that it is the lens I should have had a long ago!

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I have that lens and dont care for it but I am glad you like it I want the 50mm F/1.8

I have the 50 f/1.4 and it is for a specific use
this I just put on and shoot whatever I want however I want
(except candlelight, of course)

The Sigma 24-70mm is a close competitor to Canon's 24-70mm. I know that something is missing in the Sigma's comparing to Canon. What doesn't the Sigma offer? I can think of full time manual focusing, fully encompassed hood, and weatherproof build (and weight!). How about the degree of distortion and dark corners? Forgive my ignorance, because I'm considering it as a substitute of the expensive Canon's.

To switch to manual focusing, the focusing ring is the switching clutch. In Canon mount version, is there an additional button near the mount for AF/MF conversion? If yes, does it mean that I have to move the clutch and then push the button to focus manually?