My input is get both the Canon 17-40L and the Sigma 50mm 1.4 for about the same or less than your choices, then when you are ready/able to get your next lens, sell one of these, take maybe a $100 loss on the trade, and get the appropriate Canon upgrade for the one you sold. Then repeat for the second lens you aspire to. The key here is for all that time, you will have enjoyed about 90% of the benefit of having your cake and eating it too with both lenses, and at the end of it you basically get to the same place for about the same cost.
The Sigma lens is friggen awesome, btw. Truly stupendous. And it focuses faster than the 1.2. You may decide to suffer the indignity of not sporting the red ring and just own that sucker for good.
Also, I find that if you take reasonable care of your lenses, eBay is a great way to monetize what you don't need any more. People actually get silly with how much they will pay for used stuff, often paying more than the rebate prices for new lenses with warranties.