Wait what, Cruel and Critical rings are back?
I'm not too keen on Arans, but if I recall correctly, Cruel ring gives 5 W.Att and 5% Critical increase. Critical gives 2 (or 1, I don't remember) W.Att and 10% Critical.
This is just wild guessing, but if the Aran class have some skill that increases critical rate and damage, I think Critical would be better. If not, Cruel