what is much better? a supervisor position or be trainer?

Hi apple! It really depends what is "supervisor" in your center is. In some center, supervisor serve as the team leads or the next level after agent. In some center, supervisor is the head of all team leaders, usually one or two in the floor. The last one is better since you will be handling large number of people. But if you want less stress, try the trainer position. It really depends what's your area of expertise.

Which pays better? I would decide based on pay and enjoyment of position. Good-luck!