go for the sigma, and buy the nifty yourself, it makes economic sense lol! 
i have the 50 1.4, its a great lens no scratch that its a stellar lens! i used it tonight at a christmas parade only the streetlight was available, short winter days and all, but i bumped the iso threw it wide open and hey presto near daylight- i used it for a street parade, which the 50 on a 1.6 crop is quite good at.
What you have to remember is that its really an 80mm lens on a crop. However when it got to the firework display i had to put on the 17-40L, this is the creativeness you are after with a wide angle lens, the convenience of a zoom at the wide end- the 50 was just too cropped and wouldnt cut it, but the wide angle gave me the shot i needed.
I am just trying to give you an example of how i used my lenses. They are the only 2 i own (shamefully covers 18-55 lol), each has their strengths but sadly they each have their weaknesses.
the 50 would be great on indoor sports, but you wouldnt get the wide field of play, a wide angle zoom maybe too slow for indoors, but you could get great distortion effects and cram a whole lot more into your scene- the sigma you refer to, and the canon 10-22 are what i am currently weighing up for my next purchase.
17-40
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50mm1.4
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