Mordred = able to apply all status effect and the bonus is extra dmg deal when enemy has afflicted
Arthur = boost flat damage
i can say it's depend on your play style... if u running raid with little ppl, go Mordred and the status effect such as charm/blind will keep ur team alive and extra dmg dealt but if u running raid with all strangers+friends+unions, i would go Arthur. 1 skill from Arthur and dealt 40-50k ez MVP ez loots~
some might say Siegfried is the MVP stealer but if missed the rage/stun it's kind of hard(for me...).