If your customer is very young, I would recommend you to focus more in SnapChat. I can see lots of very young people over there. But only of your customer is nearer to the twenties than to the tens.

At least, don't choose between Instagram and SnapChat as those are two different platforms. SnapChat is about the moment as it ha sjust gone, while Instagram is about sharing the moment after adding some making up. Instagram is more creative before taking the picture while SnapChat is more "look what just happened to me".