Upvote:0
In general, as a J-1 exchange visitor, you have to follow the requirements of your J-1 program sponsor (the school or organization that issues Form DS-2019). If your J-1 program sponsor agrees, either of the options you mentioned could work:
Enter the US in December as J-1 (with December start date on DS-2019), start J-1 activities for ~2 weeks, get travel signature on DS-2019, go to France for ~2 weeks, return to US and resume J-1 activities in January.
Enter the US in December as a tourist (B-2 or VWP) but don't do any J-1 activities, then go to France, then enter the US in January as J-1 (with January start date on DS-2019) and begin J-1 activities.
On the other hand, neither of the above will work if your J-1 program sponsor doesn't agree to it. In the first case, you need them to issue a travel signature on your DS-2019. In the second case, you need them to issue an updated DS-2019 with January start date. Furthermore, if your J-1 program sponsor determines that you aren't following their rules, for example by being absent from required activities, they could end your program. As a result, your flexibility to set your schedule depends on how much flexibility your J-1 program sponsor will allow.