Summoners gain more MagPow per level than Assassins.

However, Summoners gain much less speed per level than Assassins.

Since speed determines how quickly your turn comes around in battle, a magic user raised as an assassin will be able to start using magic right off the bat. A summoner-raised character will just sit around being worthless until their turn comes around. By that time, the battle will be over, trivially close to over, or worst of all, your magic user will be dead or severely damaged. The summoner-raised character's magic will be slightly more potent, but it's wasted because their options were limited by a slow speed stat.