My little Opinion is. None. Physical Unions have High HP an can Deal Very High Damage to Single Targets with only little AP cost. Mystic Unions can deal Extreme Damage to Groups of Enemys but the have in most cases relative Low HP, an the AP Costs are high. One some Mobs you deal more DMg with Physical Arts and on other with Mystical. Some Fights are easy with a Long Range Fighting Union, and some Bosses .. ok i can go on with this for a very long Time. Play the Game the Way you like. Some fights maybe tougher since there will be a better choice for a specific fight, but i PLAY a Game for Fun.