Upload & Sell: On
I owned the first version (non-OS) of the Sigma 70-200 f/2.8 for almost six years, shooting thousands of photos of, among other things, my kids playing their games. For the price, it's a great lens.
A year ago, I decided to upgrade to the Canon 70-200 f/2.8 IS II. If the cost doesn't bother you, get it.
You can't go much wrong either way, but the Canon offering is simply spectacular, in every way. Sharp at all apertures, corner to corner (on a full frame sensor); fast AF; takes extenders remarkably well.
There's no wrong answer here, IMO, when factoring in cost. But there's no question that the Canon is a better lens, which will hold its value better over time.