Every Astel has a type in addition to class. They are Servant, Guardian, and Savior. Each type of Astel has different abilities, including summoning cost, summoning time, and skill sets. Because of this, it's wise to summon an Astel who can provide tactical advantages for the situation.
Saviors require a lot of AP to summon, but do not consume any AP while summoned. Saviors will be unsummoned when their duration ends. They have a powerful appearance skills, so it's a good idea to summon them in crucial moments. Special skills cost AP to use.
Guardians cost more AP than Servants to summon and maintain. Special skills cost AP to use.
Servants cost little AP to summon and maintain.