The different mouth pieces are more on horses preference. Some horses prefer something that has a port as it takes pressure off their tongue. Other horses like something that is single or double jointed. Some prefer something that has rollers. Really all the different mouth pieces are more on what mouth piece the horse likes. My mare likes a double jointed bit. While one of the other horses I ride prefers a single jointed. I also ride a horse who likes rollers.
If you read the tech info on the bits they actually explain them a bit better