A bit of a random first post perhaps, but I went through the process of tring to find the right bit for my horse not too long ago and let me tell you, it wasn't easy. Every horse person can tell you exactly what type of bit you should get, all based on their own preferences and personal experiences. So many people, so many opinions.
The general advise I received from friends and store assistants was to try and go with the friendliest, thickest snaffle I could find to try and keep the mouth as soft as possible. Besides of course the "financially well-off and nothing but the best will do for my horse" type of friend who advised me to get the latest design, super friendly, soft in the mouth and most expensive bit she could find. I decided to do a bit of research and I came accross an article by equine dentist Christie Dreesen, who wrote an article in Dressage magazine in the Netherlands. Here are some of her findings which helped me a lot.
The thickness of the bit is decided by the outside of the bit, where the bit touches the corners of the horse's mouth. This is where the bit puts pressure on the corners of the mouth and on the tongue. A thick bit can divide the pressure over more surface, making it more subtle. A thin bit is slightly less subtle in putting pressure on mouth. However, there are two main factors in deciding what is kind from the horse's perspective: how well does the bit fit the horse's mouth and how kind is the rider with his or her hands. To start with the rider; kindness and softness in the mouth is mainly decided by the hands of the rider. Even if the bit fits perfectly, a rough rider with hard hands can easily hurt or injure the horse.
A well fitted bit is often a thin bit. Many people ride with bits varying from 18 to 24 mm thick while some bits are up to 24 mm thick. This is too thick for most horses, causing many horses to have difficulties closing their mouths. Escpecially when the mid section of the bit is also thick or points upwards horses can develop issues as soon as the rider puts pressure on the reins. The pressure will be put on the roof of the horses mouth, causing him to open his mouth. So make sure that you look at thickness but also the shape and material of the bit.
Thicker bits aren't always friendlier on the horse. Make sure the bit fits well in the horse's mouth and don't assume all thin bits are harsher. Many horses are better off with a thinner bit that fits well.
So there it is, after two trial and errors where I bought and returned an unsuitable bit I finally chose the perfect bit for my boy Finn. It's a good fit and he responds well to it so we're happy and have started practising for the Olympics! (We're working on walk & trot at the moment but it's good to have a goals)