Haven't tried the non-IS 2.8, but I'd say you'd want to go for the one with IS (since the A7 doesn't have IBIS). I've shot with both the Sigma OS version and the Canon mark II 70-200 on Sony's NEX50M adapted with a metabones mark IV adapter and I can tell you as far as image quality is concerned there's barely noticeable differences in real world shooting.
If you are doing event or sports photography and need the AF speed, I'd strongly suggest getting the Sony's native G / G master lenses. That being said, if you just need basic AF functions, the Sigma will serve the purpose well enough. The Canon one is good though, faster (but not significantly faster) than Sigma's offering in terms of finding the focus point and locking on, just that it's pricier which personally for me, when shooting video and pulling manual focus most of the time, doesn't justify the cost.