Spells per Day

At the indicated levels, a souldrinker gains new spells per day as if she had also gained a level in a spellcasting class she belonged to before adding the prestige class. She does not, however, gain other benefits a character of that class would have gained, except for additional spells per day, spells known (if she is a spontaneous spellcaster), and an increased effective level of spellcasting. If a character had more than one spellcasting class before becoming an souldrinker, she must decide to which class she adds the new level for the purposes of determining spells per day.