Party Members share experience with their Personas, except for the Leader, which gain experience separately from own Persona.
Leader's Personas will require more Experience to get Stronger, meaning, Leader will become stronger faster than owned Personas.
Even if, the Main Character won't change Personas, that one Persona that will be used will still be lacking behind.
The best way is to Fuse and Create the most Powerful Personas available.
Although, prefer using Personas with strongest abilities.