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.
So you want the softest curb bit? I would say a mullen mouth or myler low port curb. Both spread the pressure evenly over the horse's tongue and bars. Shorter shanks are also more mild than longer ones.
A single jointed curb can create a nutcracker effect, pinching the tongue and poking the roof of the mouth. They arectually quite severe.
BTW, if you are going to use a curb bit, avoid the "Tom Thumb". Go with Luvs2ride's suggestions or even better in my opinion, a bit with a Billy Allen mouth which is arguably the most mild of any broken mouth shanked bits. Here are some examples: http://www.rods.com/dg/235,123_Billy-Allen-Bits.html
Yep, I agree with all that has been said. If you see a "curb bit" with a broken mouthpiece it is NOT a true curb bit. A "curb bit" with a broken mouthpiece is very harsh so stay away from those. I really like the way the Myler curb bits are designed. I'd look into those.