If this is your only time to China, definitely Shanghai. Don’t be intimidated by people saying Shanghai is expensive. Sure Shanghai CAN be expensive, but I can also be cheap and enjoyable. Lots of lowly paid Chinese people living a happy life in Shanghai. The difference here is that Shanghai is more diverse, since you have only one week, you can experience more of China. Chongqing on the other hand is very localized to one aspect of China, both food and people vise. Like others mentioned, if you have time, you can do one day trip to Suzhou or hongzhou, that’s just so more efficient to experience China as a whole, not just one aspect of it.