I honestly just look for what your horse is most comfortable with like my current youngster is going to be put into a straight bar flexi rubber loose ring snaffle which I believe he will be most comfortable in. My last youngster rode best in a copper ring lozenged universal bit as loved the taste and feel of the copper rings but honestly every horse is different and some may even prefare to go bitless but you will definitly know when you have found the right bit or bitless.
I usually ride my mare in a French link snaffle, but for fox hunting I need a kimberwick because she gets strong during the gallops
I try to find the right bit when I ride a new horse, depending on its need, and always looking for the softest one. If I had to choose a bit with a specific action, regarding the progression of the work I change if I feel like we don't need it anymore (for example a full cheeck snaffle).