When talking bits, it's always good to evaluate the horse that will be wearing it first, and then matching the bit to the horse.

Western bits ranging in mildest to more severe go somewhat like this:

1. Plain O ring snaffle, thick mouthpiece
2. Plain O ring snaffle, thinner mouthpiece
3. Shanked snaffle with short shanks that curve backwards slightly
4. Curb bit with low port and short shanks that curve backwards lightly
5. Curb bit with lower port and longer shanks
6. Curb bit with higher port

They coincide with a horse's degree of training pretty much. The numbers above could be used for these steps of a horse's training:

1. Beginning to train the colt, from the first time he has a bit in his mouth to where he is obeying rein cues with direct pressure.
2. For getting a little more control, a thinner mouthpiece will get a better response, once the colt understands the basics of giving to the rein pressure, you could switch to this bit. Some people use a ring snaffle on a horse for years, I would suggest moving on once the horse is responsive and rideable in all gaits.
3. The shanked snaffle will help transition a horse to a curb bit. The leverage in the shanks will aid in getting a better stop, a better head-set, and more control.
4. A curb bit with short shanks that have some curve to them will still be a pretty mild bit, and you would use one when the horse is ready to neck rein.
5. Longer shanks come later when the horse will neck rein responsively. You might never move on to a bit with a higher port or longer shanks. If your horse works great in the shanked snaffle, you can neck rein just fine with it. It's pretty much based on preference and how your horse works best.

If you are buying a horse that's already trained, find out from the previous owners what bit the horse is used to. If it's a good, comfortable bit, there's no need changing. Do what's best for the horse.