When purchasing an instructional set, it is very important to think carefully about what the exact form of use will be. Do you plan to use the set for teaching? For pleasure rides? For competitions, or as a judge? If you have determined for yourself for which situation(s) your instruction set will be used, then it is easier to make a choice. You can then make a better considered choice for a single or duo set. You also need to consider how much range you need. Another important point is the importance of two-way communication. Should the rider only be able to hear the instructor, or is it important that the rider can also say something back? Furthermore, it is advisable to read up on different brands of instruction sets.
The AXIWI instruction sets
Tips for using an instruction set