It really depends on the type of training you want to do with them. Do you want to do all positive training with no physical corrections? If so, a flat nylon buckle collar is all you need. If you want to use physical corrections ("pull & jerk" or "leash-pop" methods) there are flat nylon buckle collars, prong collars, or choke chains.
I never recommend starting with anything other than flat nylon buckle collars, and I also recommend positive training (as it is my experience that the dogs learn faster and have more fun doing it). If you have done adequate training (this means daily sessions plus ideally group classes for several months) and the dog *knows* what you want from it, and you are still having a problem with leash pulling, you may want to consider a prong collar for a time. Be sure to have a trainer who is experienced with them show you how to fit and use it. I *never* recommend choke collars in any circumstance - they cause too much damage to the dog.
This is just my opinion, of course. I know Alisha recommends starting with the prong collar and weaning off of it - I'm sure she'll explain that method to you in a little more detail. As she said in another post, we agree on the end result - we just disagree on the means to get there
