Should I immigrate after marriage or get married after immigration?

After immigration, of course. If you get a green card in the United States, you can go back to the United States freely even if you get married and divorce in China. As long as you get a green card and live in the United States for at least half a year, otherwise it may be cancelled. If you want to return to China for a long time, you'd better apply for a visa to return to the United States. But that's another story. It is best to get a green card after immigration-_-!