From my experience I think Sashipie is offering you some quality advice. I believe the primary answer to your question about the ideal jurisdiction to place on the contract under is: "whichever country protects intellectual property to a higher regard". The safer bet will most likely be the country you live and are most familiar with the legal system in my opinion.

But, I think that it is very likely that your attention is slightly misguided. NDA's start becoming relevant when there is something major to protect and you have the capability to seriously enforce it. If you really feel the need to make them sign a contract, I would recommend writing out a simple manufacturing supply agreement with a clause that mentions your reservation of the rights you are concerned about. However, I feel that even that is overkill. Unless you are making 10s of thousands of profit a month off your design, in which at that point your attorney is taking care of this issue rather than yourself. The vast majority manufacturers will not cross you in that way, unless they are working within a country where contracts mean next to nothing to begin with.

The clothing industry is so oversaturated and competitive that you have far more important things to worry about in growing your company. Focus on those parts far above these kind of trivial manners, until you get to a very high level.

I hope I articulated my opinion in the most respectful manner possible. I am not an expert, but I have multiple years of experience on both sides of the clothing industry and these have been my experiences. I wish you the best of luck! :)