Well back in 2.X determination was actually a better stat then crit for blm, now it is sort of reversed.
Crit=% of chance for damage to crit and more damage, however determination has no rng like factors and is a constant damage not reliant on % of chance.
The more crit you have the higher chance for it to happen.
What caster class are you thinking? I have not played smn for sometime so do not know how different from blm it is but blm it is WD> Spell Speed> Crit> Determination