I don't know much about GSC at all, never been much of a fan in the plant due to the lower yields usually. I don't go fully off yield, but I believe you can have quality and yield such as with the Sour Diesel you also have. The Sour D should have a good bit of a sativa high to it and speed things up a little. Never had the cut myself, can't get them where I am from.. Just going off the crosses of it and what I have read. For flowering times on the Sour D, I would say give her 70 days and she should be good to chop.. but it is going to vary a little depending on your grow conditions. I figure others take them longer, and some shorter.. but 70 days seems reasonable to me.
As for THC levels.. I figure they are kind of close.. but THC levels isn't everything. Terps play a major role in how a plant smokes and how potent it is. So a strain that has 20% thc can kick your ass more then some 24% thc strains will. But If they are both the real deal then I figure they are both hovering around 22-25%. I am sure others know more then me, but since nobody has said anything yet I figured I would chime in with my very limited knowledge on the subject.