I've just run some calculations, for AMs who have a base crit of 5%:
Critical Ring: Dmg = 1 + 0,15*1,35 = 1,2025
Magical Ring: Dmg = 1+ 0,1*1,35 = 1,135
CritRingDmg / MagRingDmg = 1,059471 (about 5,94% increase).
For the 4 matk difference to be worth more than that 5,94% increase you would need to have less than total 68 matk (which is pretty low) so for AMs the crit ring is better.
For bishops, which have that skill increase their critical change by 54%:
Critical Ring: Dmg = 1 + 0,69*1,35 = 1,9315
Magical Ring: Dmg = 1+ 0,64*1,35 = 1,864
CritRingDmg / MagRingDmg = 1,036212 (about 3,62% increase).
For the 4 matk difference to be worth more than that 3,62% increase you would need to have less than total 109 matk (which is pretty low) so for Bishops the crit ring is still better.
Note that I used the average critical damage multiplier for my calculations and have not included SE. I guess you can do the ones with SE for yourself and I doubt you'd see much difference, unless you somehow got to 100% crit chance with the magical ring.
Critical Ring: Dmg = 1 + 0,15*1,35 = 1,2025
Magical Ring: Dmg = 1+ 0,1*1,35 = 1,135
CritRingDmg / MagRingDmg = 1,059471 (about 5,94% increase).
For the 4 matk difference to be worth more than that 5,94% increase you would need to have less than total 68 matk (which is pretty low) so for AMs the crit ring is better.
For bishops, which have that skill increase their critical change by 54%:
Critical Ring: Dmg = 1 + 0,69*1,35 = 1,9315
Magical Ring: Dmg = 1+ 0,64*1,35 = 1,864
CritRingDmg / MagRingDmg = 1,036212 (about 3,62% increase).
For the 4 matk difference to be worth more than that 3,62% increase you would need to have less than total 109 matk (which is pretty low) so for Bishops the crit ring is still better.
Note that I used the average critical damage multiplier for my calculations and have not included SE. I guess you can do the ones with SE for yourself and I doubt you'd see much difference, unless you somehow got to 100% crit chance with the magical ring.