Buy a sim in France. Data roaming costs are now much cheaper in Europe, and soon voice calls will be also reduced in price. You can safely ognore the comments about a phone not working, which may have been the case with older US photnes, certainly with Apple phones everything works fine. I'd recommend Orange since they're basically the old France Telecom, and have the technical resources the other providers may not have.
Also note there are virtually no cafés, restaurants, bars, hotels, etc. that do not offer free wifi.