+1 MAG matters more but both have decreasing returns and the the weighting toward MAG decreases based on level.
Pure STR Weapons
DMG = [ATK x RANDOM(1~1.125) - DEF] x [1 + STR x (Lv+STR)/256]
STR+MAG Weapons
DMG = [ATK x RANDOM(1~1.125) - DEF] x [1 + STR x (Lv+MAG)/256]
Using some stock ATK and DEF numbers,and assuming +1 STR vs +1 MAG, plugged into an excel, around Lv10 it was +3.2% damage vs +4.5% damage (the actual damage number hitting the mob on screen), but by Lv30 it's +2.1% to +2.5%, Lv50 at +1.5% to +1.7%, Lv70 at +1.17% to +1.27%, and by Lv80 at +1.05% and +1.13%. Those exact +damage% likely aren't exact because I the amount of ATK, STR/MAG, and enemy DEF for each level could be off, but it still shows the weight change in the diminishing returns.
VIT and SPD weapons are the same way, but they weight even more heavily because instead of /256 they're /128 and /218. So 1 point of STR or SPD both give you a bigger damage boost, and likewise STR or VIT... e.g. +3.5% from 1 STR at Lv10 for an axe vs +3.2% for a katana, and +5% from +1 VIT on an axe vs 4.5% for +1 MAG on a katana; likewise like at Lv30 and 50 an extra 1 VIT on an axe is +2.6% and +1.72% vs the +2.5% and 1.71%. Because character level impacts the role of the other stat, and because it lags (e.g. your MAG is probably always 10-20 higher until the different becomes 0 at Lv99), the difference between 1 STR and 1 MAG/VIT/SPD shifts at the same rate until 1 point of each is almost (but still not exactly) the same.
Short version, prioritize MAG, VIT, or SPD, but even by around Lv15 it's a 1% difference and I think by around Lv70 it becomes a 0.1% difference.
