Should I buy an iPhone in France or usa?

You should buy it in the country where you'll be using it more; you'll likely be denied warranty service outside the country of origin(the EEA and Switzerland for countries within it).

(201460)

Since your stay in France is temporary, I would suggest that you purchase the phone in the USA. Since all iPhones sold are dual SIM-equipped, you have at least three options using it:

1. Have your USA cellular provider initialize the phone's eSIM, and get a physical SIM card from a French provider.
2. Don't use the eSIM at all, but get two physical SIM cards (one from each provider). Swap them depending on which location you are at.
3. Just get a physical SIM from your USA provider and see if they offer a "travel abroad" option. For example, AT&T does to reduce the cost of roaming charges when abroad.