Hellknight signifer spellcasting

At 1st level, and at every level thereafter, a Hellknight signifer gains new spells per day as if he had also gained a level in a spellcasting class he belonged to before adding the prestige class. He does not, however, gain any other benefit a character of that class would have gained, except for additional spells per day, spells known, and an increased effective level of spellcasting. If a character had more than one spellcasting class before becoming a Hellknight signifer, he must decide to which class he adds the new level for purposes of determining spells per day.