I've had pretty good luck with flights in September during recent years. (Notice how the flight delay thread pretty much went dormant after last August, and didn't pick up again until last month.)

Regarding PVG v. SHA, I always choose the latter when possible because it is a lot more convenient -- apart from being closer to town, much less walking is required to get to your gate. Furthermore, it has much better dining options, which come in handy during those long delays. However, my personal experience is that SHA is no star on the delay front.