I have the f/1.8 and I love it. The f/1.4 is even better in low light.
I'm not sure of the compatibility issues with yours, but I think you have to make sure that the lens has it's own autofocus motor, otherwise it won't autofocus.
I honestly hated my 1.8, maybe I just got a bad one, but my auto focus would never work. If I were you and could afford it, I'd go with the 1.4 or 1.2