It depends on the synthetic brush that is available. Generally speaking, pony hair brushes are often used in schools since they are inexpensive, yet strong and soft. However, they do not retain the shape of the brush well when wet. Synthetic brushes on the other hand can be both well suited and terrible for painting with watercolors depending on the quality (which has improved largely).
I would suggest you try both and then decide. Take the brush that can hold its shape best when wet while still being able to hold a good amount of liquid.
By the way, the best brushes for water color are usually made from mink hair (generally referred to as Kolinsky sable). For further information look here and here.