Well, from your description, i assume you've already unlocked your iphone...... If so, it doesn't make any differences whether you use it in China or America..... Just join a company whichever you like by purchasing a sim card and signing a plan..... uhhhhh... one thing you need to know is if you live in a small town or countryside, then, i recommend you to choose Verizon over AT&T due to its more reliable reception in smalltown.