Instinct= more damage with magic and healing Vitality= more hp (i think its like 7hp per point) Strength= more damage with physical attacks Speed= more speed means you can go first over attackers.
We are talking about Sonny 2 and not damage. Anyways, turns don't really matter. Mostly it's the accuracy, dodge and critical hits. You'll really notice if you use shadow blend or are in wraith form.
Instinct can help you in magic damage and healing ability. Speed is pretty useful so you don't keep missing every single attack. It also helps because the enemies will miss you more often.