Just think of it this way, would you rather win a fight or a match? if you have guys with 8 power and 1 damage how can you win if they use all their pills on someone with say 6-8 damage and win one of them. you have to have a combination. the most important factor is abilities.
Kinjo only good if the situation is good ( no stop opp ability or damage reducers) , some cards like gina glint are very good defense cards ( -4opp damage hurts ) , vermyn N only has 5 power but can be dangerous thnks to having 7 damage and a reduction ability , same with uranus her stats are crap but her ability is one of the best defenisve ones in the entire game( same goes with the two choles except obviously the 8 damage they have ) ,the main thing is to have a mixture of high power/damage cards and get defenisve ones aswell , regarding the question 6+ should be high enough for both but remeber having higer damage then power is never reali good as it would take more to win the round in the 1st place :