Summon wrenches a creature's soul from the netherworld to help the caster. If this creature kills another, it is replaced by the soul of the newly fallen victim. Higher levels allow the caster to keep the creature summoned for longer.
A summon is as strong as the creature it represents.
Recasting Summon while you have a summon out is less costly than an initial cast, making it a useful spell to cast repeatedly in order to gain Will experience. However, it does not reset the summon time.
Different creatures do not cost different amounts of mana to summon.
Kills and actions by your summon do not affect you (experience or morality).
Although the White Balverine being technically stronger than both Wraiths and Minions (Wraiths are slow and Minions don't have that much HP), the game will count them as "upgrades". This could be an error from the developers.
It is not recommended to use a White Balverine in order to kill Minions or Wraiths since it will change forever.
Since many players state that the strongest summnon is the White Balverine, it is important to know that there are five chances of getting a White Balverine as a summon. (The first that you face on the "White Balverine" main quest, three at the Arena, and another one at the Temple of Avo during the mission "The Ransom Victim" if you pay for the rescue and don't attack the assassins).