Which is better, a monthly salary of 3k in Zhenjiang or 6k in Shanghai?

It depends on the specific situation. If you don't have any relatives or friends in Shanghai and want to rent a house to come by yourself, Zhenjiang is the best place.

If Shanghai has a great chance, it will definitely develop well in the future, and I want to have more materials, then go to Shanghai.

What matters is what you think. Comfort must be as comfortable as Zhenjiang.