if u consider the diminishing effects for both card combos. it will surely deal almost same damage,
as kingring affects total matk. while when u equip red ferus it lowers ur matk but adds up damage on fire spells.
if u want higher damage. try to put 2 salamander cards on cloak. and u might wanna try using necromancer cards as part of ur combo.
however, remember that your damage will greatly be dependent on your enemy. so dont test it to only 1 player, you should conduct ur test in several players which are different in gears and jobs.