Curb and Snaffle are two different bits. A curb bit is anything with shanks, where the reins are attached to the end of the shanks. These bits require a light hand, a horse who knows how to neck rein, and very little "pull". They are fine for a soft mouth, IF you and your horse are trained to use one.
A snaffle is any bit that has the reins attached at the level of the horse's mouth and has no leverage, usually with the bridle attached to the same area. The softest snaffle is one with two joints, a french link, bean mouth, or some kind of roller in the middle. Double jointed bits lay flat in the mouth, spreading the pressure from the reins evenly.