I agree that the IS isn't a big deal and in my experience the 24-70 is a better lens (faster focusing, sharper and f2.8 is nice) definitely worth a try. I also used my 28-70 f2.8 alongside a 70-200 f2.8 and find it to be a superb combination for people photography.
However if you like the dof effect of an f2.8 prime you'll love the way that the really fast primes melt the backgrounds. Even the cheaper primes do the job very well - you could get a 50 f1.4, and 85 f1.8 and an 100 f2 secondhand for a similar price to the Sigma 85 f1.4. I've been using an 85 f1.8 for quite a while and have been very happy with the results but am now selling it as I managed to find a good deal on a used 85 f1.2, that really is a crazy lens. If you shoot wide open for head shots the dof is so thin that it's hard to get both eyes in focus. For full length shots it really makes the person stand out from the background giving a nice 3D feel. Attached are a few shots taken at f1.2, I reckon that the lens has a lot more to give than I've got from it yet. If you are ever over my way then feel free to come and play with it and the 28-70 f2.8.
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