Going on May 3rd to Beijing and Shanghai for two weeks-

Hi, it will be warm in Peking then, with the average temp of 20-22C. Sunny days will constantly fall along some drizzle days. You can take along long-sleeved shirt and long jeans. Short-sleeved shirt and shorts will help in sunny afternoon. You also need a light coat in case of weather change.

Shanghai will be hot then. The temp will vary from 20C to 32C or a little higher. Thus summer clothes are needed. Loose and casual clothes and shoes are recommended for outings. Sun-blocking cream, sunglasses and hat are also suggested to be prepared in case of strong sunshine.

Considering weather, generally it can be good travel time in both the cities. But it will be High Travel Season of China, so you may feel a little crowded then, especially for transportation.