Without question, St. Regis
In my view, from a value perspective, there is no comparison. Frankly, I'm surprised the rates are so low and that they are that close. If the Sheraton is $130, I'd expect the St. Regis to be more in the $350 to $400 range. It would be in virtually any other city. Beijing is just very competitive now in the higher end.
Location is much better at the St. Regis. You are very close to the main attractions (other than Great Wall, which is going to be a drive from either hotel) and walking distance to several things of interest.
Beijing has turned into a very busy, crowded city with horrible traffic. It will also likely be hot in September. Going home each day to the St. Regis makes a huge difference.
If you are only going to go to Beijing once, I'd recommend you do it first class and stay at the St. Regis. It's truly a bargain at the current pricing.